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This Translation First Published in 1925 



PRINTED IN GREAT BRITAIN 



PREFACE 

SUGGESTIONS have reached me from various 
quarters, that by abbreviation and excision 
of mathematical deductions and numerical 
data, my Kapillarchemie might become an introduc- 
tion to the chemistry of colloids for a larger number 
of students of medicine and technology. Since several 
books of the kind already exist, I at first viewed this 
plan with misgivings, but ultimately overcame them, 
when I realized that the facts and ideas, which to 
my mind are of special importance to colloidal 
chemistry, had not yet been presented in simple 
language. I hope that I have succeeded in doing 
this in the present book. I desire to thank Professor 
G. Barger of Edinburgh and Dr. med. R. Stern of 
Breslau for valuable hints and suggestions, and also 
Dr. L. Farmer Loeb for reading the proofs. 

H. FREUNDLICH 

DAHLEM, JANUARY, 1924 

KAISER WILHELM INSTITUTE 
FOR PHYSICAL CHEMISTRY AND 
ELECTROCHEMISTRY 



TRANSLATOR'S NOTE 

I WAS so impressed by the manuscript of Professor 
Freundlich's Grundziige der Kolloidlehre that I 
at once undertook its translation, as a token 
of my appreciation of the work, in the hope of intro- 
ducing it to a large number of English-speaking 
students. Various colleagues in this university have 
been good enough to discuss with me the rendering 
of technical terms and Dr. Edgar Stedman has read 
the proofs and made valuable suggestions, for which 
my thanks are due. 

G. B. 
UNIVERSITY OF EDINBURGH 



VI 



CONTENTS 

PAGE 

INTRODUCTION 3 

THE PHYSICO-CHEMICAL FOUNDATIONS OF 
COLLOIDAL CHEMISTRY 

A. CAPILLARY CHEMISTRY: 

1. THE INTERFACE BETWEEN A LIQUID AND A 

GAS 18 

2. THE INTERFACE BETWEEN TWO LIQUIDS . . 34 

3. THE INTERFACE BETWEEN A SOLID AND A GAS 37 

4. THE INTERFACE BETWEEN A SOLID AND A 

LIQUID 53 

5. CAPILLARY-ELECTRICAL PHENOMENA . . 75 

6. THE PROPERTIES OF INTERFACIAL LAYERS . 88 

B. THE RATE OF FORMATION OF A NEW 

PHASE 94 

C. THE BROWNIAN MOLECULAR MOVEMENT . 107 

COLLOIDAL-DISPERSE SYSTEMS 

A. COLLOIDAL SOLUTIONS, SOLS AND GELS: 

1. SOLS 113 

2. GELS 168 

B. COLLOIDAL-DISPERSE STRUCTURES OF A 

DIFFERENT KIND: 

1. MISTS AND SMOKES 191 

2. FOAMS 194 

3. DISPERSE STRUCTURES HAVING A SOLID DIS- 

PERSION MEDIUM OR HAVING MORE THAN TWO 
PHASES . . . . . . .198 

INDEX 203 

vii 



THE ELEMENTS OF COLLOIDAL 
CHEMISTRY 



INTRODUCTION 

THE view that matter is built up of indi- 
vidual, minute, sharply defined particles has 
during recent years met with conspicuous 
and acknowledged success. It was first successfully 
applied to gases, of whose properties we can give an 
excellent representation when we conceive them to be 
composed of minute particles, the molecules, swarming 
about in rapid motion. In order to give an idea of 
molecular dimensions, we may recall that the diameter 
of a hydrogen molecule, regarded as a sphere, must be 
about 0-2 millionth of a millimetre and that at o and 
760 mm. 27 trillion 1 molecules are contained in a 
cubic centimetre of a gas. Quite generally the gram- 
molecular weight of any substance contains 0-6 quad- 
rillion 1 molecules. We approach somewhat nearer 
to a realization of these numbers by the following 
illustrations. The ocean contains something like 1,300 
trillion litres, and since a single litre of water consists 

1 In accordance with a common usage, we mean here by 
billion io 12 , by trillion io 18 , by quadrillion io 24 . Hence there 
are about 600,000,000,000,000,000,000,000 molecules in a 
gram-molecular weight. 

3 



4 COLLOIDAL CHEMISTRY 

of 33 quadrillion molecules, there are many more 
molecules in a litre of water than litres in the ocean. 
If we poured a litre of water into the sea, and after 
complete mixing took another litre of water out, the 
second litre would still contain 25,000 of the original 
molecules which we had poured away. 

Van't Hoff made a great advance by showing that 
a substance is present in dilute solution in a condition 
similar to that of gases. In aqueous solution sugar 
is therefore divided up into single molecules which 
move about the liquid without any regularity, and 
almost independently of one another, like the mole- 
cules of a gas. The only difference is, that the sugar 
molecules are separated from one another by water 
molecules, whilst a vacuum separates the molecules 
of a gas. We must at once emphasize that the 
molecules of a dissolved substance, the solution mole- 
cules, do not necessarily consist of single molecules 
of the dissolved substance only ; thus in the case of 
sugar each sugar molecule certainly binds a number 
(at present unknown) of water molecules. 

The molecules consist of atoms, the smallest part- 
icles into which a chemical element can be subdivided. 
There are therefore as many kinds of atoms as there 
are chemical elements. But not even the atoms them- 
selves are uniformly filled by matter, they too have a 
definite build. Among the various theories of atomic 
structure, put forward during recent years, that 
developed by Rutherford and Bohr has met with 
special success, for it has been of the greatest assist- 



INTRODUCTION 5 

ance in interpreting the spectra of the elements, largely 
on a quantitative basis. According to this theory an 
atom may be regarded as a diminutive planetary 
system. 

An exceedingly minute, positively charged nucleus, 
with a radius of something like one billionth of a 
millimetre, is situated in the centre of the atom ; 
round it a definite number of electrons revolve in 
circular or elliptical orbits. The number of these 
electrons increases in a definite manner with the 
atomic weight of the element. The electrons are 
negatively charged particles whose mass is about 
2oW f th a t f a hydrogen atom. They are not 
only known as structural units of the atom, but also 
appear in the free state in many natural phenomena : 
thus cathode rays are streams of rapidly moving 
electrons. 

Gases, liquids,' and crystalline'solids may be regarded 
as composed of molecules and atoms. Liquids sensibly 
resemble gases, in that they too ftp$st of molecules 
in irregular motion, only th^r/frji&cules are much 
more closely crowded than tfc&c^ux ?. fhis. The special 
peculiarities of the liquic^U^aro have hiuiT+o been 
explained only to a small extent. A very ditleicnt 
case is presented by crystalline solids, such as common 
salt ; here the structural units (" bricks ") are not 
molecules, but atoms with an electrical charge, the 
so-called ions. Each ion occupies a point of a so- 
called space-lattice and executes there an oscillatory 
movement of small amplitude. We can form an idea 



6 COLLOIDAL CHEMISTRY 

of ^ace-lattice by imagining a large number of 
uniformly woven nets arranged parallel to each other 
at regular intervals. At each knot in the network 
there is an atom, in the case of sodium chloride alter- 
nately a sodium ion and a chlorine ion. The distance 
between two points of the space-lattice in sodium 
chloride is about 0-3 millionth of a millimetre. The 
mutual arrangement of the points of the space-lattice 
varies with the crystal form of the solid. 

All the structural units which have so far been 
considered, viz. molecules, atoms and electrons, are 
smaller than one millionth of a millimetre (i ^ = 
o-oooooo i mm.). There are, however, many structures 
in which we must take larger units into account, and 
these all belong to the colloids. We know there are, 
for instance, solutions which differ from ordinary ones 
in that their solute will not pass through parchment 
paper, collodion, etc. Thus if we place an aqueous 
solution of sugar or of sodium chloride in a sack of 
parchment paper, the sugar and the salt gradually 
pass through the membrane This is, however, not 
the case when a solution of gelatine or of starch is 
similarly Created. We have every reason to assume 
- ^ .A other observations confirm this that in the 
latter solutions there are much larger solution aggre- 
gates than in those of sugar and of salt. The molecules 
of these substances pass through the pores of the 
membrane, whilst the particles present in gelatine 
and starch solutions are unable to do so. In this 
way Graham was led in 1861 to differentiate solutions 



INTRODUCTION 7 

of egg white, gelatine, etc., as colloidal solutions or 
sols from the ordinary true solutions of salt and sugar. 
The term /'colloid' 1 is derived from ?5 xo'AAa, glue, 
because this substance gives a typical solution of this 
kincL^ The passage of dissolved substances through 
membranes is, called dialysis. Colloidal solutions are 
therefore characterized by the fact that they do notdia- 
lyse. Ever since Graham's time dialysis has remained 
one of the best means of recognizing colloidal solutions 
as such. If we desire to ascertain whether a solute is in 
colloidal or in true solution, we place it in a dialyser, 
such as a sack of parchment paper, hang this in a 
vessel filled with pure water and test whether or no 
the dissolved substance appears in the external liquid. 
If, even after a long time, the solute cannot be demon- 
strated outside the dialyser, the solution is colloidal. 
In this way we can for instance readily ascertain that 
the brown colouring matter of tea is a colloid, for on 
dialysis it remains inside the dialyser, whilst the sugar 
with which the tea has been sweetened, passesout. 

he_questipix.upw arises as to the size of the particles 
of a colloidal solution. The ordinary microscope does 
not reveal them, even at the highest magnification 
available. The smallest particles which can be seen 
in this way have a diameter of about ^oW mm. = 
0-5 p. Colloid particles are therefore still smaller. 
Siedentopf and Zsigmondy succeeded in 1903 in con- 
structing an apparatus, which in favourable cases 
allows of the recognition of particles having a diameter 
of one hundred thousandth of a millimetre. This 



8 COLLOIDAL CHEMISTRY 

uiiramicroscope depends on a phenonemon similar to 
that which renders visible particles of dust, when a 
powerful sunbeam enters a dark room and is viewed 
against a black background. In the ultramicroscope 
a colloidal solution, placed under the objective, is 
illuminated laterally by a powerful beam of light and 
care is taken that no portion of this light enters the 
microscope directly. The particles in the colloidal 
solution diffract the light which falls on them, just 
as the much coarser 'dust particles reflect the sunlight. 
Some of the light is scattered upwards and enters the 
microscope, thus rendering the colloidal particles 
visible. As has been pointed out, the phenomenon 
is merely similar, not identical in the two cases, for 
the minute particles present in sols cannot strictly 
speaking reflect light, as they are too small compared 
with its wave length ; they diffract it, and in this 
process the incident light is changed in_various ways/ 
With the aid of the ultramicroscope it has been 
possible to demonstrate in many sols particles having 
a diameter of 10-500 JUJLI. Not all colloidal solutions 
can, however, be resolved in this fashion ; in some the 
particles are too small, less than i /z/i ; in others the 
particles are large enough, but their optical properties 
do not differ sufficiently from those of the surrounding 
liquid. Thus gold particles in colloidal aqueous 
solution are very distinct, provided they are suffi- 
ciently large, because the optical properties of gold 
are so very different from those of water. On the 
other hand the particles of colloidal protein or starch 



INTRODUCTION 9 

solutions are generally invisible, since the solution 
molecules here contain so much water, that their 
optical properties closely resemble those of the sur- 
rounding fluid. Th^ultramicroscope is _ there! ;ore_a 
powerful aid in the recognition oj the ccdlaid^LMtoa 
ofjt .solution, but we must not infer the- absence of 
colloidal particles from the failure of the ultramicro- 
scope to reveal them. 

When Graham first distinguished colloids he believed 
that, in addition to their incapacity to dialyse, he 
coulik ascribe yet another property to colloidal solu- 
tions. He was struck by the fact, that when solutions 
of protein, starch, etc., are evaporated, the solute 
appears in the form of amorphous masses, jellies or 
varnish-like pellicles, whereas in the case of true 
solutions (of salt, sugar, etc.) well-formed crystals 
generally appear. In Graham's mind the concept 
non-crystalline, amorphous-solid therefore connoted 
that of the colloidal state. Now this connotation 
has been found to be erroneous, or rather, it is found 
to be inexpedient. The particles of many sols, such 
as gold sols, which play so important a part in colloidal 
chemistry, are crystalline, not amorphous. Nowadays 
we accordingly mean by colloid a certain state of 
subdivision of matter ; structures are termed colloids, 
when they consist of two or more phases and the 
structural units of at least one phase have a magnitude 
of 1-500 pp. It does not matter whether these struc- 
tural units are excessively large single molecules, or 
whether they are crystalline particles, amorphous- 



io COLLOIDAL CHEMISTRY 

solid particles, droplets, or gas bubbles, each con- 
sisting of many single molecules, although they are 
themselves minute. It will be remembered, that we 
mean by a phase a completely homogeneous condition, 
in which no portion can be distinguished optically 
from any other. A pure gas, free from dust or mist, 
is a homogeneous phase ; so are also pure liquids, 
solutions of truly dissolved substances and homogen- 
eous crystals. The concept " colloid " implies nothing 
whatsoever with regard to the state of aggregation of 
the particles. A clear glass cannot therefore suitably 
be called colloidal. It is indeed amorphous- solid, but 
it is uniform (homogeneous) and therefore not col- 
loidal. Nor do we therefore speak of "colloidal 
substances " as Graham did. Theoretically any sub- 
stance can be made to assume the colloidal form, that 
is to say, it can be subdivided in such a way, that it 
is not truly dissolved, but has only been subdivided 
down to particles having a diameter of 1-500 /*//. 

The more general concept disperse, introduced by 
Wolfgang Ostwald, is often useful in order to charac- 
terize the state of subdivision. Structures containing 
particles of a colloidal size, are eolloidal-disperse ; those, 
which, like milk, contain coarser particles, visible 
under the ordinary microscope, are termed cqarse- 
disperse ; true solutions may be called molecular- 
disperse. The peculiarities which characterize the 
last-named, are not indeed wholly dependent on the 
molecular-disperse condition. The fact that the 
solution molecules contain, in addition to the solute 



INTRODUCTION n 

molecule, a considerable number of solvent molecules 
is perhaps of still greater importance. In colloidal 
solutions we distinguish the disperse phase, namely 
tfiaf one, which like starch and gelatine, is subdivided 
into minute particles ; the surrounding liquid is the 
dispersion medium. The particles of the disperse 
phase are separated from one another and present 
convex surfaces to their surroundings. The dispersion 
medium is continuous and is separated from the dis- 
perse phase by concave surfaces. A similar distinc- 
tion between disperse phase and dispersion medium 
can be drawn in all colloidal structures. 

So far colloidal solutions have only been compared 
with real ones, and we conceived of them as arising 
through a continual increase in the size of the solution 
molecules. But we can approach the subject of 
colloids from another side. We have only to imagine 
that the particles of a coarse-disperse emulsion or 
suspension become so finely subdivided, that they 
are no longer visible under the ordinary microscope, 
but only under the ultramicroscope. This process is 
applied technically, when milk is rendered " homo- 
geneous/' The coarse-disperse, microscopically 
visible globules, with a diameter of about 3 ////, are 
broken up mechanically to an ultramicroscopic size 
and are hence rendered colloidal. As we shall see 
later, there are many processes by means of which 
we can pass from liquids or from solid substances 
having a coarse, continuous structure to colloidal 
solutions. It thus becomes evident, that it is really 



12 COLLOIDAL CHEMISTRY 

somewhat arbitrary to fix a superior limit to the 
colloidal state, above which the particles become 
microscopically visible. To a large extent coarse- 
disperse emulsions and suspensions also show the 
properties characteristic of colloidal solutions, only 
emulsions and suspensions are generally less stable, 
because their coarser particles settle more rapidly 
under the influence of gravity. Incidentally the 
object of rendering milk homogeneous is to delay the 
rising of the cream, that is the separation of fat glob- 
ules. With coarse-disperse structures those properties 
which are connected with the extent of the interface, 
naturally become less prominent. 

The increase in the total surface, resulting from the 
continued subdivision of matter, is indeed very import- 
ant. Let us imagine a cube with an edge of I cm. 
divided into smaller cubes each with an edge of i mm., 
then these 1,000 cubes will have a total surface of 60 
square centimetres. If we, however, make the cubes 
so small that they might be particles of a sol, if we 
give them for instance an edge of one millionth of a 
centimetre, one trillion cubes will be formed having a 
total surface of six million square centimetres, i.e. 600 
square metres. Since many colloidal solutions contain 
one per cent, by volume of colloid we readily under- 
stand what enormous interfaces may be present in 
relatively small quantities of liquid. 

At the interfaces between two phases a number of 
important chemical and physico-chemical processes 
take place : to investigate them is the task of capillary 



INTRODUCTION 13 

chemistry. Since in colloids the interface is so large, 
the chemistry of colloids is again and again concerned 
with capillary-chemical phenomena. It will there- 
fore be convenient to discuss these first with reference 
to similar, smaller interfaces, directly susceptible of 
observation and measurement. In the following 
account a section on capillary chemistry will therefore 
precede the discussion of colloids. 

Capillary chemistry does not, however, comprise 
the only set of physico-chemical phenomena, of which 
a knowledge is required for the understanding of the 
chemistry of colloids. The fact that Graham at first 
regarded the amorphous-solid state as a real means of 
differentiating colloids, shows that the state of aggre- 
gation of colloidal particles is important. We must 
therefore decide, how we can distinguish amorphous- 
solid from crystalline, and in particular we must 
inquire into the conditions which determine the appear- 
ance of a new phase. The preparation of many col- 
loidal solutions consists in the production of colloidal 
particles in a true solution, in the shape of a difficultly 
soluble precipitate in a sufficiently fine state of 
division. 

A third set of phenomena is related to those pro- 
perties which connect colloidal solutions with true 
ones. We have already pointed out that the mole- 
cules of gases or of a dissolved substance are subject 
to rapid motion, that of heat. Now if we observe the 
particles of a sol under the ultramicroscope, or the 
coarser particles of an emulsion under the ordinary 



14 COLLOIDAL CHEMISTRY 

microscope, we see that these particles do not remain 
at rest, but are subject to an incessant, lively, dancing 
movement, which recalls that of a swarm of gnats. 
This phenomenon was observed as early as 1827 by 
the English botanist Robert Brown, in the case of fine 
powders suspended in water, for instance, granules 
contained in pollen grains of plants, and after him it 
is called the Brownian movement. It has been estab- 
lished, that this movement is indeed due to the heat 
motion of the molecules of the liquid, which collide 
with individual colloidal particles; the haphazard 
impacts produce the movement, somewhat as the 
kicks of football players determine the movements 
of a football. The phenomena connected with the 
Brownian movement may also suitably be discussed 
before we turn our attention to the colloids themselves, 
especially since the most important laws governing 
this movement were first discovered by means of 
coarse emulsions. 

In the following survey we will therefore first -discuss 
the physico-chemical foundations of the theory of 
colloids, indicated above, and then construct on this 
basis the theory of colloids themselves. Hitherto we 
have only referred to colloidal solutions or sols as 
examples of colloidal-disperse structures and to 
emulsions and suspensions as examples of the coarse- 
disperse. These are, however, by no means the only 
ones with which the subject of colloids is concerned. 
We obtain a survey of the simplest possibilities, when 
we couple in each case two states of aggregation and 



INTRODUCTION 15 

remember that either of these may figure both as 
dispersion medium and as disperse phase. In the 
following table the first-mentioned state of aggregation 
will be regarded as the dispersion medium, the second 
as the disperse phase. We then have : 

Gaseous-liquid : Mists 
Gaseous-solid : Smokes 
Liquid-gaseous : Foams 

Liquid-liquid: Emulsions ( When colloidal-dispersc 
T ..,,., o -{ these constitute colloidal 

Liquid-solid : Suspensions { solutions> sols or gels< 

Solid-gaseous : Solid foams 
Solid-liquid : Solid emulsions 
Solid-solid : Solid suspensions 

It will be seen, that such common formations as 
mists, smokes and foams belong to the subject of 
colloids. The gels are also important and comprise 
not only jellies, but also fibres, membranes, etc. Gels 
were formerly regarded as solid emulsions, i.e., col- 
loidal-disperse droplets imbedded in a solid frame- 
work. More recent investigations have rendered this 
view improbable ; they are more likely sols, in which 
solid particles are so abundantly present, that the 
dispersion medium, the liquid, is reduced to very thin 
films, which, like a foam, separate the closely packed 
particles. Solid suspensions are represented by many 
glasses ; thus for instance the beautiful gold ruby- 
glass owes its colour to colloidal-disperse gold 
particles. 

It may cause some surprise that in the above manner 



16 COLLOIDAL CHEMISTRY 

we pick out for detailed consideration a particular 
state in the subdivision of matter and a particular 
range in the size of particles. Nor has this procedure 
passed without challenge. Nevertheless I regard it 
as justified, even if it were only on the ground that so 
very many structures must be accounted colloidal. 
Without exaggeration it may be said that the vast 
majority of liquids and solids present in organized 
nature, belong to the colloids. All body fluids, cell 
sap, blood, lymph, etc., are colloidal solutions ; 
they contain proteins as well as other substances in a 
state of colloidal division. All solid substances such 
as cell- walls, walls of blood vessels, muscle fibres,, 
nerve fibrils, have the character of gels. Hence the 
chemistry of colloids has also an important bearing 
on all technical processes concerned with products 
derived from living matter. In tanning, dyeing, the 
production of fibres and of rubber, the preparation of 
food stuffs, etc., colloidal-chemical phenomena have 
always to be considered. The theory of colloids will 
not only remain one of the essential foundations of 
biology and physiology ; it will also continue to be of 
decisive importance to technology. 



THE PHYSICO-CHEMICAL FOUNDA- 
TIONS OF COLLOIDAL CHEMISTRY 

A. CAPILLARY CHEMISTRY 

r I ^HE subject of capillary chemistry may be 

I divided into natural subdivisions, according 

-* to the nature of the interfaces which separate 

the various possible pairs of phases. We can thus 

distinguish the following interfaces : 

liquid gaseous 
liquid liquid 
solid gaseous 
solid liquid 
solid solid 

For each pair of phases it is especially the surface ten- 
sion, or more generally, the interfacial tension, a quantity 
characteristic of the interface, and its relation to 
chemical properties, which require discussion. On 
account of the complete rigidity of the interface 
between two solids, the section relating to this pair drops 

2 17 



i8 COLLOIDAL CHEMISTRY 

out, but in addition to those dealing with the other four 
pairs of phases, a fifth section will have to be devoted 
to capillary-electrical phenomena, in which electrical 
influences are connected with capillary-chemical ones. 
A sixth section will deal with the thickness of the 
interfaces. 



1. The Interface between a Liquid and a Gas 

THE SURFACE TENSION OF PURE LIQUIDS 

In the case of the liquid-gaseous interface, which will 
be discussed first, we are dealing with surface tension 
sensu proprio. This concept seems at first to present 
some difficulties to the understanding, and yet the 
phenomena which suggested it are of common occur- 
rence and easy to observe. Thus small drops of a pure 
liquid, when not subjected to external influences, tend 
to assume a completely spherical shape. This is readily 
seen with drops of mercury, or of water falling on a dry, 
dusty floor or on a cabbage leaf. If the drops are large, 
they become flattened by gravity. For the same reason 
a falling drop of water assumes the well-known pear 
shape. A different kind of external influence shows 
itself, when a drop of water is placed on a clean glass 
plate. The drop does not remain spherical, but spreads 
and wetting occurs, as the result of forces acting between 
the glass and the water and preventing the assumption 
of a spherical shape. This wetting is seen very dis- 
tinctly, when a drop of alcohol is placed on clean glass ; 



LIQUID AND GAS 19 

the alcohol does not remain as a sphere, but at once 
spreads in a thin layer over the whole plate. On the 
other hand a dusty surface, or a plant leaf with a waxy 
coating, cannot be wetted by water, and then the liquid 
assumes that shape which is not influenced by any 
external force, namely a spherical one. Now the sphere 
is characterized by the fact, that for a given volume it 
has a minimum surface. If we were required to give 
to a fixed quantity of liquid the smallest possible sur- 
face, we should have to make it spherical. Since 
liquids, in the absence of an external force, spontane- 
ously assume a spherical shape, we may conclude that 
they have inherent in them a tendency to make their 
surface as small as possible. It is then only a small 
step to conceive of this tendency as residing in a 
membrane, which envelops the liquid wherever it 
is in contact with a gas. The tension of this mem- 
brane is then the so-called surface tension of the 
liquid. The comparison to a stretched indiarubber 
membrane, which at once suggests itself, may only 
be instituted with caution : the tension in indiarubber 
changes with the degree of stretching, but a pure 
liquid has a constant surface tension at any part 
of its surface, no matter how large the surface may 
become. 

Now the great variety of shapes which a liquid can 
assume, whether as drops, as jets or as menisci, can be 
explained on the assumption, that the liquid tends 
to acquire a minimum surface in contact with a gas. 
Interesting experiments, based on familiar phenomena, 



20 COLLOIDAL CHEMISTRY 

are described in " Soap-bubbles and the Forces which 
Mould Them/' by C. V. Boys. We mention one 
example. The hairs of a dry brush, when separated 
in air, remain so under water, but are drawn together 
into a bundle with a much smaller surface, as soon as 
the brush is taken out of the water. The surface ten- 
sion brings about a minimum interface between water 
and air. 

From the great variety of shapes of a liquid surface 
there results a correspondingly large variety of methods 
by which the surface tension may be measured. Thus 
the latter may be deduced from the shape of jets issuing 
from a non-circular orifice, from falling drops, from the 
wave length of ripples produced by a vibrating tuning 
fork, from the weight and the curvature of a hanging 
drop, etc. Here we will only deal with two such 
methods. The heavier the drop which a liquid forms 
in slowly issuing from a tube, the greater is its surface 
tension. Since water has a relatively high surface ten- 
sion, drops of water are larger than those of ether 
formed under similar conditions, for ether has a lower 
surface tension. This will readily be understood when 
we congfder, that the drop hanging from the end of the 
tube is kept attached to the liquid inside by surface 
tension, whilst gravity tends to tear it off. The larger 
the surface tension, the larger the drop which can still 
be retained. The weight of a drop, or the number of 
drops furnished by a given volume of liquid, is often 
determined by the stalagmometer (Fig. i) introduced 
by I. Traube. The liquid is sucked into the pipette- 



LIQUID AND GAS 



21 



like tube and the number of drops is determined, which 
flow out between two marks a and b. The weight of 
a single drop can be calculated from the volume between 
the two marks and the density of the liquid. At K a 
length of capillary is inserted, in order to secure a 
sufficiently slow formation of drops. If this is not 
done, the^size of the drops not only depends on the 
surface tension and on gravity, but also on the kinetic 
energy, with which the liquid issues* 
from the orifice. The stalagmometer 
is chiefly used for comparative ex- 
periments, and is calibrated by means 
of a liquid of known surface tension. 
Another common method of deter- 
mining surface tension is that of the 
capillary rise. This depends on the 
readily observable phenomenon, that 
when a capillary, for instance of glass, 
dips into a wider vessel, the water 
attains a much higher level inside 
the capillary than outside. This 
phenomenon of course gave the name " capillarity " 
to the whole subject of surface actions, and thus 
also led to the term capillary chemistry. In order 
to understand this ascent of a liquid inside a nar- 
row tube, we must consider the wetting process a 
little more fully. If a drop of water be placed on a 
clean glass plate, it spreads and extends to a thin 
membrane ; in that case we speak of complete wetting, 
A droplet of mercury, on the other hand, remains 




K 



FIG. i. 



22 



COLLOIDAL CHEMISTRY 



AB 



spherical and does not spread ; here there is no wetting 
at all. Other liquids, of an oily nature, form lenticular 

masses, so that we can 
speak of a definite angle, 
the angle of contact. This 
is the angle a in Fig. 2, 
formed between the 
FlG 2 liquid-gaseous and the 

solid-liquid interface. 

With complete wet ting the angle of contact is o, with- 
out any wetting at all it is 180. Now when a clean 
capillary tube is dipped 
into water, the water 
spreads over the whole 
inner surface of the tube, 
because glass is completely 
wetted, i.e. over the area 
indicated in Fig. 3 by the 
letters ABEF, and over the 
whole of this area water is 
therefore in contact with 
air. Surface tension tends 
to diminish this consider- 
able surface, but diminu- 
tion can only take place 
by the water being drawn 
into the capillary. If FlG - 3- 

this occurs up to the 

level GH, the whole surface EFGH is eliminated, 
and the sensibly smaller surface ABGH remains 



LIQUID AND GAS 23 

as the interface with air. The height to which the 
liquid ascends is limited by the action of gravity and 
an equilibrium is reached as soon as the force of surface 
tension, action upwards in the direction of the arrow, 
is balanced by the weight of the liquid in the capillary. 
For the same radius of the capillary the surface tension 
is proportional to the height of the capillary rise and to 
the density of the liquid. More exact mathematical 
considerations lead to a formula, which enables us to 
calculate the surface tension from the capillary rise. 

With a non-wetting liquid, such as mercury, we find 
in analogous fashion, that the level inside the capillary 
is depressed below that of the liquid outside. For 
since mercury does not wet the glass, a layer of air 
would remain between the liquid and the inner surface 
of the tube, and to this extent mercury would be in 
contact with air. Surface tension reduces this air- 
surface and the mercury is depressed inside the capil- 
lary. The above simple conception only applies to 
completely wetting and to non-wetting liquids. With 
incomplete wetting, when there is an angle of contact 
between o and 180, the relationship between capillary 
rise and surface tension is less simple. 

The force, which surface tension exerts on a liquid, 
is not very large ; thus for water at 18 it amounts to 
74 mg. over a distance of i cm. What this means will 
be made clearer by imagining, as did Clerk-Maxwell, a 
frame (Fig. 4) of thin wire, of which the lower bar CD 
can slide up and down and supports a small scale pan. 
In this frame we suspend a lamella of water. (It does 



COLLOIDAL CHEMISTRY 



not affect the argument, that this is difficult to accom- 
plish with pure water ; it can readily be done with a 
lamella of soap solution, in which case the relationship 
is, however, more complex, since the soap solution is 
colloidal.) The lamella attempts to contract, and the 
force which is thus exerted, amounts, if CD be exactly 
I cm. long, to double the surface tension, for this ten- 
sion acts on the front as well as on the back of the 

lamella. At 18 we would 

A rs 

therefore have to place 
2 x 74 = 148 mg. on the 
scale pan, in order to balance 
the tendency of the lamella 
to contract. This train of 
thought also explains how 
the diminution of the surface 
must be regarded as an effect 
of surface tension. As a 
rule the surface tension is 
not expressed in milligrams 
weight per centimetre, but in so-called absolute 
units. We must then remember that the weight of 
1,000 mg. equals 981 dynes. The table on page 25 
gives the surface tension of a few liquids in dynes per 
centimetre. 

In general it may be said that mercury, molten 
metals and molten salts have a large surface tension ; 
water also has a relatively large one ; for organic 
liquids it is smaller, for liquefied gases still smaller. 
The surface tension is therefore generally large, when 



FIG. 4. 



LIQUID AND GAS 25 

the boiling-point is much above room-temperature, and 
small in the opposite case. 

TABLE I 
Surface Tension of Liquids 



Liquid. 
Mercury . 
Water . 
Benzene . 
Ethyl alcohol 
Ethyl ether 
Chloroform 
Carbon bisulphide 



Temperature. Surface Tension. 



15 
18 

20 
20 

20 

20 
20 



436 
73 

28-8 
22-0 

16-5 

26-3 
33-5 



The surface tension of pure liquids almost invariably 
decreases with rise of temperature, but the rate of 
decrease is by no means the same for all liquids. For 
water it amounts 'to about 2 per cent, per degree. 

There still remains to be considered a quantity which 
is closely related to the surface tension, namely the 
free surface energy. Since the surface tension tends to 
make the surface as small as possible, it is evident, that 
work will have to be expended in enlarging the surface. 
We have only to imagine that in the arrangement, 
shown in Fig. 4, we wish to enlarge the lamella ; this 
will require work, for the tendency of the surface ten- 
sion to make the lamella smaller will have to be over- 
come. The amount of work which will have to be 
expended, is greater, the greater the surface tension 
and the greater the area by which the lamella is to be 
increased. It is equal to the surface tension multiplied 
by this area. This work is simply the free surface 
energy. Conceptually it is related to surface tension 



26 COLLOIDAL CHEMISTRY 

in the same way as work (which is the product of a 
force and a distance) is related to a force. The free 
surface energy is a quantity which always tends to 
assume a minimum value ; for the area of the surface, 
which increases and decreases with it, also tends 
towards a minimum as the result of surface tension. 

The above consideration of a definite physical quan- 
tity and the inquiry into the conditions under which 
this quantity assumes a minimum (or maximum) value, 
may appear unfamiliar to some readers ; in theoretical 
physics, this way of regarding problems has, however, 
proved very fruitful. 

THE SURFACE TENSION OF SOLUTIONS 

An examination of the surface tension of solutions 
leads to their separation into two groups. In some the 
surface tension differs but little from that of the pure 
solvent ; generally it is a little higher ; this group 
comprises the aqueous solutions of salts, particularly 
of inorganic ones ; further the solutions of many sub- 
stances in organic solvents, such as benzoic acid, cam- 
phor and naphthalene in alcohol or ether. Such solu- 
tions are called capillary-inactive or surface-inactive. 
The second group, of the capillary-active or surface- 
active solutions, is characterized by the fact that the 
surface tension of the solvent is lowered by the solute, 
often very strongly even at small concentrations. This 
group contains almost exclusively aqueous solutions of 
many organic substances, such as alcohols, aldehydes, 



LIQUID AND GAS 27 

fatty acids, acetone, amines, esters. In some cases, 
such as the solutions of many organic salts, hydroxy- 
acids, etc., in water, the assignment to either group is 
somewhat arbitrary. These substances, however, lower 
the surface tension of water but slightly. 

The following important rule applies to the capillary- 
inactive solutions of inorganic salts in water. As is 
well known, these salts are largely (probably even 
wholly) dissociated in solution into cations and anions ; 
in a solution of sodium chloride we have therefore only 
Na'-ions (the cations) and Cl'-ions (the anions). Simi- 
larly a lithium chloride solution will contain Li'-ions 
and Cl'-ions. Now it has been found that all lithium 
salts, no matter from what acids they are derived, 
increase the surface tension of water more than the 
corresponding sodium salts and these more than 
potassium salts. Sulphates, independently of the 
nature of the cation, increase the surface tension more 
than chlorides, and these again more than bromides, 
etc. This behaviour is an additive one. The char- 
acteristic series of cations and anions so obtained 

Li > Na > K 
S0 4 >Cl>Br>I>CNS 

are called the lyotropic series. They play an important 
part in a large number of other natural phenomena. 
By lyotropic properties of salts we therefore mean 
the additive properties expressed by the above series. 
Probably the lyotropic behaviour is an expression of 
the affinity of the ions towards water. For it may be 



28 COLLOIDAL CHEMISTRY 

taken as certain, that every ion binds a considerable 
number of water molecules, and is to a certain extent 
therefore surrounded by an aqueous envelope. Unfor- 
tunately we have as yet no reliable means of measuring 
this power of binding water, the degree of hydratlon 
of the ions. If we had such means, we should at once 
be able to elucidate quantitatively a large number of 
phenomena which at present remain more or less 
obscure. Such scanty and rather uncertain measure- 
ments of the hydration of ions, as have been made, point 
however to the conclusion, that the Li'-ion is more 
strongly hydrated than the Na'-ion, and this more 
strongly than the K'-ion. Among anions the sulphate 
ion is more strongly^ hydrated than the Cl'-ion, and 
this more than the Br'-ion. The lyotropic series there- 
fore also applies to the hydration of the ions, 

The other group, of capillary-active solutions, also 

shows a remarkable regularity. A comparison of the 

solutions of closely related substances, belonging to a 

homologous series, like that of the fatty acids, formic, 

acetic, propionic and butyric, shows that the lowering 

of surface tension produced by them increases strongly 

as we ascend the series. Fig. 5 shows this behaviour. 

Here the concentrations C of the aqueous fatty acid 

solutions in gram molecules per litre have been plotted 

as abscissae against the surface tensions a as ordinates. 

It will be seen that formic acid lowers the surface 

tension of water but slightly, acetic acid about 2 to 3 

times as strongly, propionic acid again 2 to 3 times as 

much. This behaviour has been observed in this series 



LIQUID AND GAS 



29 



up to undecoic acid, with n carbon atoms. A quite 
similar behaviour, namely a great uniform increase in 
the lowering of surface tension while ascending a 




036 



0-54 



FIG. 5. 



homologous series, has been observed with many other 
organic substances, such as aldehydes, esters and 
amines. This regularity is referred to as Traube's 



30 COLLOIDAL CHEMISTRY 

rule; its explanation will not be attempted until 
later (p. 93). 

There are quite a number of phenomena in which 
the marked lowering of the surface tension of water 
by small quantities of capillary-active substances comes 
into play. Thus this lowering causes the vigorous 
movements shown by water on coming into contact 
with ether vapour ; the lowering of the surface tension 
does not take place uniformly, and so liquid is drawn 
from places of low surface tension to those with a 
higher one. A similar local inequality of surface ten- 
sion is also the cause of the rapid movements shown by 
particles of camphor on a clean water surface. Move- 
ments of the same kind are shown by many surface- 
active and volatile substances, such as benzoic acid, 
atropine, fragments of flowers and leaves containing 
essential oils (chamomile flowers, mint and rosemary 
leaves). The so-called " tears " formed by strong 
wines are likewise due to the great change in surface 
tension, resulting from changes in the concentration of 
a capillary-active solution. The upper part of the 
glass, when not quite full, is merely wetted by the 
liquid. On account of its great surface this wetting 
layer evaporates rapidly and jthe alcohol disappears 
first. A liquid with less alcohol remains behind, which, 
on account of its greater surface tension, contracts to 
drops which run down the sides of the glass like tears, 
while neighbouring portions become wetted again by 
fresh liquid derived from the main bulk, 



LIQUID AND GAS 31 

ADSORPTION AT LIQUID GASEOUS INTERFACES 

Capillary-active solutions show a property which at 
first sight may seem rather trivial, yet is of fundamental 
and general importance. These solutions are apt to 
froth. The formation of a stable foam is a complicated 
phenomenon, which can only be discussed more fully 
later on (p. 195). Here we merely say that a mini- 
mum surface tension favours frothing, but the liquid 
must not be too volatile, nor too mobile. Hence ether 
and benzene do not froth, although their surface ten- 
sion is small. Capillary-active aqueous solutions on the 
other hand do froth, because their surface tension is 
sufficiently small and the films of liquid which con- 
stitute the foam do not evaporate too rapidly, nor 
collapse too readily. Now if foam be generated on an 
aqueous solution of amyl alcohol, and then rapidly 
separated from the bulk of the liquid, and subsequently 
the amyl alcohol content of both portions be deter- 
mined, we find that the liquid resulting from the foam 
is richer in amyl alcohol than the main bulk. Experi- 
ments of this kind may be made in a semi- quantitative 
fashion by letting air bubbles of known size ascend in 
a capillary-active solution and securing, that the upper 
part of the liquid, in which they break up, only com- 
municates with the rest through a narrow tube. Any 
change in concentration, which is thus set up, cannot 
be abolished by convection or diffusion. Under these 
conditions we also observe the transference of the 
dissolved substance by means of the gas bubbles. 



32 COLLOIDAL CHEMISTRY 

This can only be explained on the assumption that 
the dissolved substance collects on the surface of the 
bubbles so that the portion of the liquid which im- 
mediately adjoins the bubble contains more of the 
solute than the rest of the liquid. The theorem, put 
forward above, that the free surface energy always 
tends towards a minimum, renders this phenomenon 
intelligible. For the free surface energy is equal to 
the product of the surface tension and the area of the 
surface. In pure liquids only the latter can change, 
for the former has a definite fixed value at any given 
temperature. The case of a solution is different, for 
here not only can the area of the surface change, but 
also the surface tension, which may increase or decrease 
through a change in the concentration of the solution. 
If the surface tension falls with increase in concentra- 
tion, the free surface energy would become a minimum, 
if the solute collected on the surface. Its concentration 
would increase there, and since the surface tension is 
determined by the concentration in the surface layer, 
the surface tension, and with it the free surface energy, 
would decrease. The theorem that the free surface 
energy tends towards a minimum hence leads to the 
result, that a dissolved substance must increase its 
concentration at the surface, if it lowers the surface 
tension ; conversely it must decrease its concentration 
at the surface, if it raises the surface tension. 

Such an increase in the concentration of a solute 
at the interface between a liquid and a gas, is an 
example of adsorption, by which we mean the loose 



LIQUID AND GAS 33 

fixation of a substance at an interface. The above- 
mentioned theorem, connecting the lowering of sur- 
face tension with the increased concentration of a 
substance on the surface, is termed Gibbs* adsorption 
theorem after the American theorist Willard Gibbs, who 
first formulated it. We can approximately calculate 
the quantity adsorbed in the experiment described 
above, on the bubbling of air through a capillary-active 
solution. For instance, in the case of a 0-0003 normal 
solution of nonoic acid in water it amounts to about 
O'oooi mg. per square cm. of the surface generated by 
the gas bubbles. This quantity may appear negligible, 
but we must remember that, as has already been 
pointed out, a colloidal-disperse substance may develop 
a surface Of many millions of square centimetres ; 
considerable quantities of substance may therefore be 
adsorbed in this manner. 

We may further point out, that the theorem em- 
ployed above, according to which the free surface 
energy tends towards a minimum value, is merely a 
special case of the more general rule, that the so-called 
free energy always tends to become a minimum. This 
latter rule in its turn is merely an expression of the 
so-called second law of thermodynamics, which applies 
generally to equilibria in all kinds of natural phenomena. 
Although experiments demonstrating the increase of 
surface concentration in a capillary-active solution 
are not very numerous, we may nevertheless depend on 
them, since such an increased concentration is merely a 
necessary result of the second law of thermodynamics. 

3 



34 COLLOIDAL CHEMISTRY 

2. The Interface between Two Liquids 

The surface of contact between two partially or 
wholly immiscible liquids presents phenomena quite 
similar to those met with in the case of a liquid and a 
gas. We know that when a small quantity of chloro- 
form or of petrol is shaken with water, the drops of 
these liquids suspended in the water are spherical, and 
accordingly the same considerations apply as in the 
case of the interface between a liquid and a gas. We 
must therefore postulate an interfacial tension, which 
endeavours to give to the drops of the one liquid a 
minimum surface. The interfacial tension can thus be 
measured by the same methods as the surface tension, 
and the values obtained are of similar magnitude to 
those found for the surface tension. On the whole 
they are perhaps somewhat smaller. 

We again encounter the antithesis between capil- 
lary-active and capillary-inactive solutions, and also 
Traube's rule, when we examine the interfacial ten- 
sions between aqueous solutions of organic substances 
and liquids which are not completely miscible with 
them. A complication must, however, be considered, 
namely the solubility in the second liquid of the sub- 
stances dissolved in the water. As long as the solute 
is principally present in the aqueous solution, and thus 
resembles that at a liquid-gaseous surface, Traube's 
rule is very evident. But if the solute mostly passes 
from the water to the other liquid, the lowering of 
interfacial tension is generally much smaller than we 



TWO LIQUIDS 35 

might expect, and Traube's rule scarcely applies, if at 
all. Thus butyric acid has been found to lower greatly 
the interfacial tension between water and olive oil, in 
the same way as it lowers surface tension, but alcohol, 
which is capillary-active at the interface between water 
and air, has but a slight effect on the interfacial tension 
between water and olive oil, since it is soluble in the 
latter. 

The interface between two liquids is likewise subject 
to Gibbs' adsorption law, and here the experimental 
verification is somewhat easier than at a liquid-gaseous 
interface. We can for instance investigate the change 
in the concentration of a solution, caused by a stream 
of falling droplets of mercury. We then find, in 
accordance with Gibbs' law, that those substances are 
most strongly adsorbed, which most lower the inter- 
facial tension between mercury and water. 

The question naturally suggests itself : what is the 
relation between the surface tensions of two liquids and 
their interfacial tension ? The relationship between 
these three quantities determines whether one liquid 
will spread on the surface of another, in experiments 
analogous to those on wetting. If we for instance 
place a drop of alcohol on a clean water surface, the 
alcohol rapidly spreads with a lively movement over 
the whole surface of the water. The diagrammatic 
representation in Fig. 6 will explain how this takes 
place. Let A be the water and B the alcohol. The 
surface tension of the water acts in the direction PA, 
that of the alcohol in the direction PB, and the inter- 



36 COLLOIDAL CHEMISTRY 

facial tension between water and alcohol in the direction 
PC. Since alcohol is miscible with water in all pro- 
portions, we might question whether such an inter- 
facial tension really exists ; its existence must, how- 
ever, be inferred, at least in the first moments of the 
contact between the two liquids. For a jet of alcohol, 
issuing from an orifice under water, behaves in the 
neighbourhood of the orifice exactly like a jet of ben- 
zene, which, being not completely miscible with water, 
certainly has an interfacial tension. Since the inter- 
facial tension between two completely miscible 

liquids, such as alcohol 
and water, is in any case 
small, the larger surface 
tension of water, acting 
FlG 6 along PA, will always 

preponderate over the 

surface tension of alcohol acting along PB ; the liquid 
at P, and therefore the drop, is drawn out over the 
water surface. We thus obtain the general result 
that a liquid, which is completely miscible with 
water, and has a smaller surface tension than water, 
must spread over a water surface. 

The case is not so clear, when the second liquid is 
incompletely miscible with water and therefore pos- 
sesses a distinct interfacial tension in contact with the 
latter. In that case the surface tension of water, act- 
ing along PA, may still preponderate over the surface 
tension B of the second liquid and the interfacial ten- 
sion, so that the second liquid still spreads over the 




SOLID AND GAS 37 

water, as is the case with ether. If, however, the inter- 
facial tension is large, the case may occur that the 
tensions along PB and PC together compensate for the 
tension along PA. The second liquid then remains 
lying on the surface of the water as a lenticular drop ; 
this happens for instance with pure petroleum. Often, 
especially with oleic acid and impure petroleum, a thin 
film of the second fluid first spreads on the surface of 
the water. This so greatly lowers the surface tension 
of the latter that a further quantity of the second 
liquid remains in a lenticular form ; the diminished 
surface tension of the water, still acting along PA, now 
balances the two other tensions. The thin films of 
petrol, spread on water, cause, as is well known, the 
beautiful iridescent colours, depending on interference, 
which may be seen when a motor car has been standing 
on a wet pavement. 

3. The Interface between a Solid and a. Gas 

THE SURFACE TENSION OF SOLIDS 

It is the mobility of liquids which enables us to 
recognize and measure their surface tension, but when 
we come to consider the interfaces of solid substances 
we encounter quite different conditions. Here the 
particles suffer mutual displacement with great diffi- 
culty, so that we cannot recognize the surface tension 
directly or measure it. Nevertheless it is found ex- 
pedient to assume a surface tension of solids against 
a gaseous space, 



38 COLLOIDAL CHEMISTRY 

This applies in the first place to amorphous-solid 
substances such as glass, pitch and resin. As will be 
discussed more fully later, these may be regarded as 
extremely viscous liquids : their structural units are 
probably molecules, arranged without any pattern, 
like those of a liquid, but they can only move extremely 
slowly with respect to one another. In the long run 
they do move, however. Thus a long horizontal glass 
rod, which is only supported at its extremities, sags in 
the course of time ; the contents of a barrel of pitch, 
as used in road mending, may often be seen to have lost 
their original shape and to begin to flow when the staves 
of the barrel have been removed. For the same reason a 
thread of glass, hanging down vertically, will be length- 
ened by gravity at a temperature at which glass still 
behaves like a solid, provided that the thread is long 
enough and heavy enough. On the other hand, if the 
thread is not so long, surface tension may preponderate 
and shorten the thread in order to reduce its surface. 
Indeed, the surface tension has been deduced from the 
length at which the thread neither extends nor con- 
tracts ; thus for lead glass at 500 a value of 70 
(measured in dynes per centimetre) has been found for 
the surface tension, which is therefore of the same order 
of magnitude as that of liquids. 

In the case of crystalline solids a similar comparison 
with liquids cannot however be instituted. Crystals 
are built up in a manner quite different from that of 
liquids, as has already been indicated, and as will be 
shown more fully later; their structural units are 



SOLID AND GAS 39 

atoms or ions, situated with strict regularity at the 
points of a space lattice, where they execute small 
oscillatory movements. We might at first imagine 
that under these conditions there can be no question 
of a surface tension, and yet powerful arguments tell 
in favour of its existence. Their comprehension re- 
quires some further explanation. 

In dealing with the surface tension of liquids we 
emphasized that the free surface energy always tends 
to assume a minimum value. The free surface energy 
is large, when the surface is large. A dew of very fine 
droplets accordingly has a larger free surface energy 
than when these droplets coalesce to a single drop. 
Hence there must be a tendency in this direction and 
such droplets are actually known to unite when they 
come into contact ; this is particularly evident in the 
case of droplets of mercury, when touching each other, 
since mercury has a large surface tension. Now, as 
was pointed out, this principle of the free surface 
energy is only a particular case of the corresponding 
principle of the free energy and thus of the second 
law of thermodynamics. This principle requires that 
the above tendency of the free energy to assume a 
minimum value will show itself by any means which is 
at all possible. When the droplets are not in direct 
contact, they are nevertheless indirectly connected 
through their vapour, if they are in the same space 
and are not separated by a wall. Therefore small 
droplets will unite to larger ones, even if they are not 
in direct contact, because the smaller drops evaporate 



40 COLLOIDAL CHEMISTRY 

more rapidly than the larger ones. Liquid therefore 
escapes from the smaller ones as vapour and is con- 
densed on the larger, until in the end all the smaller 
drops are consumed in this manner. The final state 
is represented by the same single large drop which 
would have been formed if the droplets had been placed 
in direct contact. 

This train of thought therefore leads to the result 
that small drops must have a larger vapour pressure 
than large ones and that the increase in the vapour 
pressure is the more considerable, the greater the sur- 
face tension of the liquid in question. The convex 
surface is most strongly curved in the smallest drops ; 
therefore there is an increase in the vapour pressure of 
a liquid with increased convexity of surface and con- 
versely there is a lowering of vapour pressure, when the 
surface of the liquid is concave, as for instance in a 
capillary. The increase in the vapour pressure only 
becomes appreciable (10-100 per cent, of the ordinary 
vapour pressure) in the colloidal-disperse region, e.g. 
with droplets having a radius of o-ooooi cm. or less. 

Experiments, devised to demonstrate the increase in 
the vapour pressure of small droplets, have hitherto 
been neither numerous nor sufficiently exact. We 
must remember that such experiments are easily ren- 
dered fallacious by slight variations of temperature, so 
that the liquid simply distils from a warmer to a colder 
place. Yet we need have no doubt regarding this 
phenomenon, since it depends on the general applica- 
bility of the second law of thermodynamics, Now a 



SOLID AND GAS 41 

phenomenon quite similar to that observed with drop- 
lets, is found in a deposit of minute crystals, i.e. they 
too have a vapour pressure greater than that of large 
crystals. Thus by passing air through finely powdered 
^>-dichloro-benzene a larger vapour pressure was at 
first observed than afterwards, when all the smallest 
crystals had evaporated. Similarly it has been shown 
experimentally that deposits of minute crystals in a 
vacuum unite in course of time to larger ones. Per- 
haps, however, these experiments are also invalidated 
by the temperature not being rigorously uniform and 
constant. From the higher vapour pressure of smaller 
crystals we can deduce that they must also have a 
lower melting-point than larger ones ; Tammann and 
Meissner have actually shown that various solids, when 
in the form of thin lamellae (thinner than o-ooi mm.), 
melt at a lower temperature than when they are in 
larger fragments. 

Just as the increased vapour pressure of minute par- 
ticles is related to the surface tension of liquids, so we 
may also connect the phenomena depending on the 
increased vapour pressure of minute crystals with a 
surface tension of crystalline solids. We would there- 
fore, on the basis of these observations, postulate such 
a surface tension for solids. 

A few phenomena may still be mentioned which can 
be explained on this hypothesis. Since surface tension 
tends to reduce the surface to a minimum, it should 
also tend to round off the sharp corners and edges of a 
crystal. Such crystals with rounded edges and corners 



42 COLLOIDAL CHEMISTRY 

are actually known. If, for instance, we heat a piece 
of metal to a high temperature which is still appreci- 
ably below the melting-point, surface tension may 
begin to overcome the rigidity of the individual crystals 
constituting the metal, the so-called crystallites, which 
then become rounded at the edges and corners. Sur- 
face tension is also the cause of recrystallization, in 
which the small crystals unite to larger ones at such 
temperatures below the melting-point. Probably this 
phenomenon is similar to the union of minute droplets 
to larger ones, when in direct contact, for it is unlikely 
that the formation of larger crystals could be due to the 
increased vapour pressure of the smaller ones bringing 
about a distillation ; the process takes place far too 
rapidly to allow of the latter explanation. Further 
the difference between iron-nickel meteorites and ordin- 
ary iron-nickel alloys may be explained on the assump- 
tion that the meteorites have been formed from 
ordinary alloys by such a recrystallization. 

THE ADSORPTION OF GASES BY SOLIDS 

We have seen, that at the interface between a liquid 
and a gas the surface tension can easily be measured, 
but that an adsorption cannot so easily be demon- 
strated. The interface between a solid and a gas pre- 
sents the opposite case. As was pointed out in the 
preceding section, the surface tension of crystalline 
solids has not yet been measured, although there are 
good reasons for assuming that such a surface tension 
actually exists. On the other hand we know many 



SOLID AND GAS 43 

phenomena which must be interpreted as adsorption 
at a solid-gaseous interface. With solids there is no 
difficulty in securing a large surface : some can be 
reduced to an extremely fine powder ; others, such as 
charcoal, are very porous by nature and have a corres- 
pondingly large surface. The colloidal-disperse fine 
structure, characteristic of organized matter, may be 
preserved by cautious carbonization, and thus we 
obtain in vegetable and animal charcoals solid sub- 
stances with an enormous surface. Such charcoals, 
among which coco-nut charcoal must be specially 
mentioned, bind gases by loose combination. The 
gases may readily be removed by evacuation and heat- 
ing ; their fixation has long been known as adsorption. 
During recent years adsorption has claimed special 
attention, since it was extensively applied during the 
war. The box respirators of gas masks contain as 
principal constituent granulated charcoal, capable of 
vigorous and rapid adsorption, which completely 
retains poison gases, present in the inspired air. The 
following experiment illustrates the adsorption of gases. 
A calibrated glass tube, closed at its upper end, is 
filled with ammonia gas and dips into a trough of 
mercury. A piece of charcoal is heated to redness, 
cooled and introduced through the mercury into the 
tube, so that it floats on the surface of the metal. The 
volume of the gas rapidly diminishes and the mercury 
rises in the tube. The ammonia which has disappeared 
is fixed on the charcoal and can be recovered by ex- 
haustion or by heating. The following experiment is 



44 COLLOIDAL CHEMISTRY 

even more striking. A current of coal gas is passed 
through benzene, so that it becomes saturated with 
the vapour, and passes on to a burner, where it burns 
with a luminous, sooty flame. If, however, a layer of 
adsorption charcoal, only a few centimetres in length, 
is interposed before the burner, the benzene is adsorbed 
and the flame becomes non-luminous. The recovery of 
gaseous benzene and of other organic vapours by means 
of adsorption on charcoal has recently acquired con- 
siderable technical importance. 

In these cases a definite adsorption equilibrium is 
established. A given variety of charcoal, at given 
temperature and pressure, takes up a definite quantity 
of gas per gram of charcoal. If the pressure be 
lowered, the charcoal parts with some of this gas ; if 
the pressure is lower from the beginning, the quantity 
adsorbed per gram of charcoal is also smaller ; if the 
pressure be raised, a further quantity of gas is taken 
up. If we return to the original pressure, we find again 
the same amount adsorbed as was present originally. 
By plotting the pressures p (in centimetres of mercury) 
as abscissae, and the quantities adsorbed (in cubic 
centimetres at o and 760 mm.) as ordinates, we obtain 
a curve like that represented in Fig. 7. Such a curve 
is termed an adsorption isothermal, since it represents 
the behaviour of adsorption at a given temperature. 
It is characterized by the fact, that the quantity 
adsorbed does not simply increase proportionally to the 
gas pressure, but that at low pressures adsorption is 
relatively much greater than at high ones. The quan- 



SOLID AND GAS 



45 



tity adsorbed does indeed increase with rise of pressure, 
but much less rapidly at high than at low pressures. 
The solid adsorbing substance is called the adsorbent, 
the gas which is adsorbed is termed the adsorptive, and 



I 



160 



120 




FIG. 7. 

the combination of adsorbent and adsorptive is known 
as the adsorbate. 

On comparing the adsorption of different gases by 
one and the same adsorbent, we find that a gas is the 
more strongly adsorbed, the more readily it is condens- 
able. Difficultly condensable gases, like oxygen and 
nitrogen, are very slightly adsorbed at room tempera- 
ture ; carbon dioxide, ammonia and ethylene are 
adsorbed much more strongly. Hydrogen is generally 



46 COLLOIDAL CHEMISTRY 

adsorbed more strongly than its slight condensability 
would suggest, whereas helium and neon are only 
adsorbed very slightly, in accordance with expectation. 
Argon resembles nitrogen in adsorbability ; the adsorp- 
tion isothermal of the former gas is represented in 
Fig. 7. In any case this loose union of argon with 
charcoal can hardly be regarded as a chemical com- 
bination in the ordinary sense, since no chemical com- 
pounds of argon are known. This does not imply, 
that the adsorption must be regarded as a " physical 
combination/' sharply differentiated from a chemical 
union. Adsorption does, however, belong to some 
such class of loose combinations, as are attributed in 
chemistry to the action of subsidiary valencies. 

Nothing very definite can be said about the influence 
of the adsorbent. Charcoal, whether vegetable or 
animal, greatly exceeds in adsorptive power all other 
adsorbents hitherto examined. Recently the dried gel 
of silicic acid has also been found to be a very powerful 
adsorbent. Infusorial earth (Kieselguhr), pumice and 
meerschaum are much weaker. Pretty generally an 
amorphous substance and minute crystals seem to 
adsorb weight for weight more powerfully than the 
finest powder of the same substance, prepared by grind- 
ing larger crystals. On account of the technical im- 
portance of charcoal as an adsorbent, much attention 
has been paid to the properties which determine a high 
adsorptive power. It seems that the cellular structure 
of the wood must be preserved as far as possible by 
a gentle treatment during carbonization, so that the 



SOLID AND GAS 47 

original large cellular surface is changed as little as 
possible and the charcoal does not sinter. We must 
further ensure that no difficultly volatile tarry sub- 
stances coat the surface. It is best to carry out the 
carbonization in such a way that the formation of tar 
is restricted as much as possible from the outset. 

With rise of temperature adsorption always dimin- 
ishes. In order to obtain considerable adsorption the 
temperature must therefore be lowered. This is the 
basis of the use of adsorption to produce a vacuum. 
The last traces of vapours, such as that of mercury, 
may be very effectively removed by putting the space 
to be exhausted into communication with a vessel, 
cooled in liquid air or hydrogen, and containing a good 
adsorption charcoal. 

The following effect of temperature must also be 
taken into account. As is well known, there exists for 
every gas a definite so-called critical temperature, above 
which the gas cannot be liquefied. At higher tempera- 
tures the gas cannot therefore be present in an adsorp- 
tion layer in the liquid state, but only as condensed 
gas, whereas below the critical temperature it may also 
be present in liquid form. If the temperature is suffi- 
ciently low, we may therefore assume that every gas will 
form a thin liquid film in direct contact with the 
adsorbent. 

The second law of thermodynamics, which has re- 
peatedly been referred to, leads to the result, that a 
process (such as the evaporation of a liquid) which is 
promoted by rise of temperature, uses up heat, whereas 



48 COLLOIDAL CHEMISTRY 

a process, which is checked by such a rise, gives out 
heat. In accordance with this rule, heat is given out 
by the adsorption process, because this process is 
checked by rise of temperature. The evolution of heat 
may be shown by means of a thermometer immersed 
in an adsorbent, which is then allowed to take up a 
gas. This heat of adsorption is responsible for an effect 
occasionally described : in attempting to measure the 
body temperature by inserting into the mouth a ther- 
mometer bulb wrapped in dry flannel, quite fallacious 
temperatures up to 44 may be registered, because the 
flannel adsorbs water vapour and in so doing gives out 
heat, There is a quantitative connexion between the 
decrease of adsorption, due to rise of temperature, and 
the heat of adsorption, so that the latter may be cal- 
culated from the decrease in the adsorption. 

If an adsorbent is in contact with two gases at the 
same time, they mutually displace each other. Both 
gases are adsorbed, but each somewhat less than if it 
were present by itself. The more strongly adsorbable 
gas preponderates in the mixture, even if its partial 
pressure is small. If this were not so, the charcoal 
of a respirator could not remove small quantities of 
poison gas, for the air is present in large excess. The 
more easily condensable poison gas is, however, 
adsorbed much more strongly than air, even although 
it only represents a minute fraction of the gases passing 
through the respirator. 

The question now suggests itself, whether the 
adsorption of a gas may be regarded as a surface 



SOLID AND GAS 49 

condensation in accordance with Gibbs' law already 
referred to. At present we cannot definitely affirm 
this, for we have not yet succeeded in measuring the 
surface tension of solids and its dependence on the 
gas pressure. Unless this be achieved, we cannot 
calculate the amount of the adsorption from the surface 
tension, and test its agreement with the amount 
observed experimentally. All we can say is that in 
general the characteristics of gaseous adsorption are 
not at variance with what might be expected from a 
surface condensation according to Gibbs. Since we 
can at present make no progress along these lines, a 
different conception, due to Haber and Langmuir, 
has been preferred of recent years. Attention is 
directed to the attraction which the surface molecules 
of the adsorbent exert on the gas. Such an attraction 
may be assumed to exist on the following grounds. 
As has been pointed out, the ions of a solid crystalline 
salt (Na'- and Cl'- ions in the case of sodium chloride) 
are situated at the points of a space lattice. It has 
further been concluded, that the crystal is held to- 
gether by the attraction which the ions exert on one 
another and that this attraction is simply the force 
which was previously regarded as chemical attraction. 
Now the ions on the surface of the crystal only adjoin 
points of the space lattice on one side, towards the 
interior, thereby differing from the ions inside the 
crystal, which are surrounded on all sides. The 
attraction exerted by the surface ions is only balanced 
towards the interior ; a certain residuum extends into 

4 



50 COLLOIDAL CHEMISTRY 

the gas space and is responsible for the adsorption. 
This conception may be extended to all solids, and 
with its aid the dependence of adsorption on tempera- 
ture has been successfully calculated from the con- 
densability of the gas. 

ADSORPTION CATALYSIS OF GAS REACTIONS 

Gaseous adsorption is specially important because 
of its significance in many gas reactions ; at a low 
temperature nearly all such reactions are influenced 
by it. If, for example, molecules of methane collide 
with oxygen molecules in a gas space at ordinary 
temperature, the number of methane molecules 
oxidized is altogether negligible, but if the two gases 
are passed over finely divided platinum, which adsorbs 
them, a vigorous combustion takes place. This is 
applied in automatic gas lighters, which contain 
platinum black. Adsorption catalysis is also used 
in many technical gas reactions, as in the production 
of sulphur trioxide according to Knietsch, when 
sulphur dioxide and oxygen are passed over platinum, 
or in Haber's synthesis of ammonia, in which hydrogen 
and nitrogen flow over a finely divided alloy of iron. 

The processes with which we are here concerned 
are very various. Two examples of an acceleration 
due to adsorption may be considered more fully. In 
the first place the decomposition of stibine according 
to the equation : 

4SbH 3 = 4Sb + 6H, 

The liberated antimony forms a mirror on the walls of 



SOLID AND GAS 51 

the reaction vessel. It adsorbs the readily condens- 
able stibine stfongly, but the difficultly condensable 
hydrogen only slightly. The course of the reaction 
may be interpreted and calculated quantitatively on 
the assumption, that only those stibine molecules are 
decomposed, which are adsorbed on the surface of the 
antimony. The fact that the antimony separates 
from the very beginning as a mirror on the glass, more- 
over shows that the reaction takes place preferentially 
on the surface of the glass. 

The conditions of this reaction are specially favour- 
able to acceleration, since the hydrogen formed is but 
slightly adsorbed and does not, as it were, remain 
adhering to the surface of the adsorbent, while the other 
product, solid antimony, actually favours the reaction. 
In many other reactions the products are, however, 
strongly adsorbable ; they form a surface layer, 
which checks further catalysis. This occurs, for 
instance, in the above-mentioned formation of sulphur 
trioxide from the dioxide and oxygen, according to 
the equation : 

O a = 2S0 8 



Of these gases SO 3 is the most readily condensable 
and accordingly the most strongly adsorbed ; the union 
of S0 a and O 8 is indeed accelerated in the adsorption 
layer, but a layer of SO S is soon formed there and checks 
the access of the other gases to the platinum surface. 
The velocity of the reaction finally depends on the 
rate with which the gases diffuse through the thin 



52 COLLOIDAL CHEMISTRY 

layer of SO 8 . Many other readily condensable gases 
act like sulphur trioxide, forming adsorption layers 
on the surface of the adsorbent or changing it by an 
ordinary chemical reaction. Thus when Knietsch's SO 8 
synthesis was applied technically, the elimination of 
the last traces of arsenic compounds from the reacting 
gases at first caused difficulties since arsenic checks 
the reaction to an extraordinary extent, and " poisons " 
the catalyst. 

Among the numerous gas reactions taking place 
at interfaces, the following further examples may be 
mentioned : Catalysis of the reaction between oxygen 
and hydrogen at many surfaces (platinum, quartz, 
porcelain), the decomposition of nickel carbonyl into 
nickel and carbon monoxide at nickel surfaces, the 
decomposition of carbon monoxide into carbon dioxide 
and carbon at surfaces of nickel and of cobalt, the 
formation of phosgene from carbon monoxide and 
chlorine by passing these gases over charcoal. The last 
named is the technical method for preparing phosgene. 

Until recently the strong condensation, to which 
gases in the adsorption layer are subject, was considered 
a sufficient explanation of the extraordinary accelera- 
tion of gas reactions. A numerical estimate shows, 
however, that this condensation can hardly account 
for the phenomenon. We shall rather have to assume 
with Polanyi that hydrogen, oxygen, nitrogen, etc., 
dissociate much more strongly in the adsorption layer 
than in the gas space at the same temperature ; it is 
therefore the larger content of atoms of these gases in 



SOLID AND LIQUID 53 

the adsorption layer, which may explain the increased 
reaction velocity at a surface. 



4. The Interface between a Solid and a Liquid 

THE INTERFACIAL TENSION OF SOLIDS AGAINST 
LIQUIDS 

Conditions are here again similar to those at the in- 
terface between solids and gases. The interfacial ten- 
sions and their variations are difficult to determine. On 
the other hand adsorption is very pronounced and can 
easily be measured. Nevertheless approximate deter- 
minations of the interfacial tension have been possible 
in a few cases, by means of the following phenomena. 
Small particles of a solid have a greater solubility than 
larger crystals of the same substance, just as small 
particles and droplets have an increased vapour pres- 
sure. This increased solubility may be connected 
quantitatively with the interfacial tension, and its 
existence may be directly demonstrated. If a satur- 
ated gypsum solution be placed in contact with 
ordinary gypsum crystals, there is of course no change. 
But if to such a solution extremely minute gypsum 
crystals be added, having a diameter smaller than i /*, 
some gypsum dissolves, since solubility is increased by 
extreme disintegration. The process of solution may 
be recognized by the streaks which appear as the 
result of the difference in the refractive indices of 
layers of different concentration ; such streaks are 
often seen when sugar dissolves in water, Since there 



54 COLLOIDAL CHEMISTRY 

is a method, the streak method of Topler, which allows 
of the detection of exceedingly minute differences in the 
refractive power and thus of the resulting streaks, it 
has been found possible to detect also the very slight 
streaks formed when minute gypsum crystals are 
dissolved in a saturated solution. Quantitative deter- 
minations of this increased solubility of gypsum and 
barium sulphate as granules of known size, have given 
for the interfacial tension values of about 1,000 dynes 
per centimetre, much higher therefore than those 
known for the surface tension of liquids. 

This increased solubility is the cause of the enhanced 
reactivity often shown by solids, when in a state of 
very fine division. Laboratory experience shows that 
if solids are intended to react, they must be carefully 
powdered. Since in general no increase in solubility 
is observed until the diameter of the particle falls 
below i fji t the solid must at least be ground to this 
degree of fineness. This consideration has probably 
been applied in the so-called leucolith process, recently 
described ; under ordinary conditions anhydrite or 
anhydrous calcium sulphate reacts with water so slowly, 
that it does not set to a cement, as does plaster of 
Paris ; by grinding it to an extremely fine powder a 
useful cement may, however, be obtained. 

The very limited possibility of measuring the sur- 
face tension of solids and their interfacial tension 
against liquids, is particularly unfortunate in relation 
to the elucidation of the phenomena of wetting. These 
have already been briefly alluded to, and will now 



SOLID AND LIQUID 



55 



be considered somewhat more fully. If a drop of a 
liquid B, represented in Fig. 8, lies on a solid plate 
A, three forces act at the point P. The surface 
tension PA of the solid substance A tends to draw 
the latter in the direction PA, whereas the surface 
tension PB of the liquid B, and the interfacial tension 
PC between the solid and the liquid, act in the opposite 
direction and both attempt to hold the drop together. 
If the surface tension of the solid largely preponder- 
ates over the other two tensions PB and PC, the liquid 
will be spread over the 
solid, and complete wetting 
occurs. Obviously wetting 
is also favoured, when the 
tensions PB and PC are as 
small as possible. Com- 
plete wetting therefore 
often occurs with liquids of 

small surface tension, such as alcohol and ether. 
Water, which has a fairly large surface tension, on 
the other hand does not readily wet a large category 
of substances, mostly organic, but comprising also 
many inorganic crystals. Among the most difficult 
to wet are waxed, polished or smoked metal plates, 
and many leaves of plants, such as those of Mimosa 
or the cabbage. The extent to which leaves are 
wetted must be taken into account in the spraying of 
crops with insecticides. Some solids, such as quartz 
glass, are readily wetted by water, which fact pu 
bably depends on the interfacial tension between JdTes 



FIG. 8. 



56 COLLOIDAL CHEMISTRY 

solids and water being small ; silicates have a pro- 
nounced affinity for water. 

The angle between PB and PC in Fig. 8 is the so- 
called angle of contact. For those liquids which only 
wet a solid incompletely, this angle has not such 
a definite value as we might indeed expect. Its 
magnitude depends greatly on the previous treatment 
of the surface, on the duration of contact with the 
liquid, etc. The cause of this behaviour is unknown. 
An important factor is certainly that on prolonged 
contact the liquid penetrates more deeply into the solid, 
and that this affects the angle of contact. It is only in 
this way that so-called halation figures can be explained, 
such as are formed on a clean glass plate, to which a 
trace of grease has been applied locally, e.g. by touch- 
ing with a finger tip. If we breathe on the plate, the 
droplets of water on the greasy spot have a quite 
different appearance from those on the rest of the 
plate ; the greasy glass is wetted incompletely, and 
on it the drops have irregular, indented contours, 
whereas the clean portion is either wetted completely 
or covered by drops with regular borders. That these 
halation figures may be formed again, even after 
vigorous cleaning, must be attributed to a deep pene- 
tration of the grease into the glass. 

A phenomenon, very similar to wetting, is shown 
in the displacement of one liquid from the surface of 
a solid by a second liquid. We have only to imagine 
that in Fig. 8 the gas space is occupied by another 
liquid, and to assume that PA is the interfacial ten- 



SOLID AND LIQUID 57 

sion of the solid against the first liquid, PB the inter- 
facial tension between the two liquids, and PC the 
interfacial tension of the solid against the second 
liquid. All the other conditions remain unaltered. 
Then there are three possibilities : the first liquid may 
displace the second, and the latter remains on the 
surface in the form of lenticular drops, or the second 
liquid may displace the first and the drop spreads 
completely over the surface, or finally the second 
liquid remains lying on the surface with an angle of 
contact, as is shown in Fig. 8. 

Instead of examining these displacement pheno- 
mena at a solid surface, we can also test the behaviour 
of a fine powder, when brought into contact with the 
two liquids ; in the latter case the final result is the 
same. If we introduce a little powdered quartz into a 
stoppered cylinder containing water and supernatant 
benzene, and shake, the quartz collects in the aqueous 
layer, because water displaces benzene from the 
surface of the quartz. On the other hand red oxide 
of lead collects in the benzene layer, or settles on the 
benzene-water interface, because benzene in this case 
either displaces the water, or forms an angle of contact 
with it. A choice between the two last-named alter- 
natives cannot be made by experiments like the above, 
but we can decide with some degree of certainty, 
whether a substance is hydrophillc, like quartz, or 
hydrophobic, like red lead. 

Wetting and displacement are processes of import- 
ance, both in nature and in the arts, This is so in 



5 8 COLLOIDAL CHEMISTRY 

the highest degree with the metallurgical process of 
flotation, which during recent decades has met with 
wider and wider application. This process depends 
on the fact that sulphide ores, such as galena, zinc 
blende and copper pyrites, are much less readily 
wetted by water than is quartz, and this difference is 
rendered more pronounced, when they are slightly 
greasy, or covered with a thin layer of oil. A sulphide 
ore may therefore be separated from its siliceous 
gangue by placing the finely powdered ore in water, 
adding some oil and blowing a powerful current of 
air through the liquid. The gangue remains in the 
water, whilst the sulphide ore forms a stable froth 
which can be scooped off. Often the hydrophily of 
the gangue does not by itself suffice for the separation, 
and in the presence of oil the gangue also enters the 
froth. This may, however, be prevented by the 
artifice of adding a little acid, which so increases the 
hydrophily of the gangue, that the latter passes wholly 
into the water, whilst the hydrophobic properties of 
the sulphide ores are not sensibly diminished. 

Among natural phenomena the ingestion of solids 
by amoebae and the formation of the exoskeleton of 
protozoa is according to Rhumbler largely regulated by 
displacement processes ; in the latter case he reports 
the existence of very regular angles of contact. 

ADSORPTION IN SOLUTION 

The adsorption of dissolved substances closely 
resembles that of gases ; it has been known for a long 



SOLID AND LIQUID 59 

time and is easily observed. If for instance we shake 
blood charcoal with an aqueous solution of methylene 
blue, the solution at once loses much of its colour. 
Although in no way colloidal, methylene blue is a dye 
and at the same time a salt, and since salts show 
peculiarities with respect to adsorption, it is better to 
illustrate such adsorption by means of a non-electrolyte. 
A similar strong decrease in the concentration is 
observed, when an aqueous solution of amyl alcohol 
is shaken with charcoal. Acetone in water, or ben- 
zoic acid in benzene, behave in the same way. Here 
also undoubted equilibria are established. The 
adsorbed substance can be washed out by adding 
solvent and to a given concentration in the solution 
there corresponds a definite quantity of the adsorptive 
on the charcoal, no matter whether the adsorption 
takes place in dilute solution from the beginning, or 
whether the dilution is only reached later by adding 
the solvent. Here also the adsorption isothermal 
defines the relationship between concentration in the 
solution and the quantity adsorbed, in much the same 
way as with gases (see Fig. 9), that is, at small con- 
centrations relatively more is adsorbed than at larger 
ones. Fig. 9 relates to the adsorption by blood char- 
coal ; the abscissae c are concentrations in equivalent 
weights per litre, the ordinates a the quantities 
adsorbed in milli-equivalents per gram of charcoal. 
Here also the equilibrium is as a rule rapidly established, 
often in a few seconds or minutes, if the adsorbent 
is not very dense, nor the adsorptive of very high 



6o 



COLLOIDAL CHEMISTRY 



molecular weight, so that diffusion to the interfaces in 
the interior of the adsorbent is not delayed. 

The adsorption isothermal, as represented in Fig. 9, 
only applies to dilute solutions. At higher concen- 
trations the quantity adsorbed approximates to a 
certain constant value, the saturation value, and at 




extremely high concentrations a seems to decrease 
again. It is only in dilute solutions, that the quantity 
of substance adsorbed at the interface changes so 
much more rapidly than the concentration in the 
solvent, so that the behaviour resembles that of 
adsorbed gases. At higher concentrations the solvent 
enters into serious competition with the solute, and 
since as a rule we can only measure the changes in 



SOLID AND LIQUID 61 

the concentration of the solute, but are unable to 
determine the adsorption of the solvent, there is 
considerable uncertainty concerning the true quan- 
tities of the solute and solvent which are adsorbed, 
and this also renders the course of the adsorption 
isothermal more complicated at high concentrations. 

In the case of adsorption from solution, the con- 
nexion with surface-tension phenomena is more 
clearly noticed than in gaseous adsorption ; this is 
what one would expect, if the former kind of adsorp- 
tion is a surface condensation, according to Gibbs' 
law. Thus Traube's rule also applies here. It will 
be remembered, that in surface-active solutions the 
lowering of surface tension increases strongly and 
regularly, when we ascend a homologous series of 
organic substances (cf. p. 29). From this it can be 
deduced that the quantity adsorbed on the surface 
of the liquid must increase in a corresponding manner. 
In the adsorption of organic substances from aqueous 
solution we indeed find, that in accordance with 
Traube's rule, adsorption increases strongly and 
regularly, when we ascend a homologous series. Fig. 
10 illustrates this behaviour by means of the adsorp- 
tion isothermals of some fatty acids. As before, the 
abscissae c represent the concentration of the solution 
in gram-molecular weights (mols) per litre, and the 
ordinates a the quantities adsorbed in milligram- 
molecular weights (millimols) per gram of charcoal. 

Just as the surface-activity and the adsorbability 
resulting therefrom change, so does also the solubility 



62 



COLLOIDAL CHEMISTRY 



of the members of a homologous series ; the solubility 
decreases in ascending theseries, albeit not with the 
same degree of regularity which characterizes Traube's 
rule. There is, however, generally some sort of 

connexion between 
adsorbability and 
solubility, in as much 
as substances which 
are readily soluble 
in a given solvent, 
are but little ad- 
sorbed from that sol- 
vent by charcoal and 
other adsorbents, 
whereas sparingly 
soluble substances 
are adsorbed more 
abundantly. In 
accordance with this, 
aromatic substances 
like phenol, benzoic 
acid and aniline, are 
almost always ad- 
sorbed from aqueous 
solutions more 
strongly than aliphatic substances. The great differ- 
ence between readily soluble maleic acid and sparingly 
soluble fumaric acid is also expressed in their adsorb- 
ability : fumaric acid is adsorbed much more strongly 
than its stereo-isomeride. 




Fio. 10. 



SOLID AND LIQUID 63 

The assumption, that adsorption is a surface con- 
densation according to Gibbs, leads to an interpreta- 
tion (which cannot, however, be succinctly reproduced 
here) of the fact, that adsorption is usually strong in 
liquids with a high surface tension, such as water and 
concentrated sulphuric acid, and is much weaker in 
organic liquids having a low surface tension. This 
assumption (of a surface condensation) further accords 
well with the circumstance, that the order of adsorba- 
bility of a series of substances is not greatly affected 
by substituting one adsorbent for another ; it is 
sensibly independent of the nature of the adsorbent. 
Thus the order of adsorbability of the series w-octyl 
alcohol^ sec.-octyl alcohol^ heptyl alcohol]> tribu- 
tyrin> acetone was preserved unchanged in testing 
three different adsorbents, viz. blood charcoal, talcum 
and sulphur. Of these three charcoal is by far the 
most active adsorbent, being 500 times as active as 
the next one, talcum. 

Apart from the phenomena already discussed, 
adsorption in solution is considered to include a 
further group having a quite different character. The 
difference between the two groups becomes particularly 
evident when we employ as adsorbents substances 
which may be regarded as difficultly soluble salts (kaolin 
is an example) and when these adsorbents are allowed 
to take up electrolytes. Suitable electrolytes, which are 
strongly adsorbed, are certain dyes, like methylene 
blue, strongly ionized in aqueous solution and devoid 
of a colloidal character. Now the adsorption of 



64 COLLOIDAL CHEMISTRY 

methylene blue by kaolin differs in various ways from 
the adsorption of non-ionized or slightly ionized sub- 
stances by charcoal and analogous adsorbents. One 
difference is that in the former case only one ion is 
adsorbed, the dye-stuff cation, whilst the anion 
(generally a Cl'-ion, since we are mostly concerned 
with methylene blue chloride) remains behind in the 
solution in approximately unchanged concentration. 
Now in solution a definite number of anions must of 
course correspond to an equivalent number of cations. 
In place of the adsorbed dye-stuff ion other cations are 
found in the solution, and these are derived from the 
adsorbent ; in the case of kaolin Ca"-ions are chiefly 
involved. This kind of adsorption is therefore termed 
an exchange adsorption and is polar in character. In 
contradistinction to it, the adsorption of non-elec- 
trolytes described above, will be termed an apolar 
adsorption. The designation " polar " is chosen on 
account of the electrical antithesis between anion 
and cation, both of the adsorbent and of the adsorp- 
tive. We may imagine the exchange adsorption to 
take place as follows : the silicate ions of the kaolin 
constitute a solid framework, because they are, so to 
speak, difficultly soluble. The cations of the kaolin, 
especially the Cations, are mobile, i.e. they can wander 
out of the surface molecules into the solution, provided 
that other cations, such as the dye-stuff ions in the 
above example, can replace them in the silicate frame- 
work. 
It is possible, that in this exchange adsorption we 



SOLID AND LIQUID 65 

are simply concerned with the same valency forces 
as unite the ions of a salt, and that the dye-stuff ions 
displace the cations of the kaolin, which formed part 
of the solid surface itself. It is, however, also con- 
ceivable that the cations of the kaolin are from the 
outset not all within the solid framework, but that 
some arc in the adsorption layer ; in this case they 
would still be close to the silicate ions of the solid 
surface, and in the exchange adsorption, it would be 
the cations of the adsorption layer, which are replaced 
by dye-stuff ions. There are, however, various indi- 
cations, that in exchange adsorption the behaviour 
of the valency forces is rather different from their 
behaviour in apolar adsorption. Thus the capacity of 
an adsorbent for apolar adsorption is largely indepen- 
dent of its capacity for polar adsorption, as revealed 
by exchange adsorption. The one kind of capacity 
cannot be deduced from the other. Thus 12 grams 
of kaolin are equivalent to i gram of blood charcoal 
in respect of the adsorption of methylene blue, whereas 
even 1,000 grams of kaolin are insufficient to produce 
the effect of I gram of blood charcoal in the adsorption 
of heptyl alcohol. A similar disparity appears, when 
we consider the simultaneous adsorption of several 
adsorptives. In apolar adsorption there is generally 
a displacement (crowding out), as in the case of the 
adsorption of a gaseous mixture (see p. 48) ; the more 
strongly adsorbable substance largely occupies the 
surface and the amount of the less adsorbable sub- 
stance is thereby greatly diminished. Similarly in 
5 



66 COLLOIDAL CHEMISTRY 

exchange adsorption one cation can crowd out another, 
and one anion a second anion. On the other hand the 
exchange adsorption of an electrolyte is scarcely, if 
at all, influenced by a non-electrolyte. Cases are 
moreover known, where an adsorbent apparently 
adsorbs in both apolar and polar fashion. Thus it 
has been found in the adsorption of dyes by charcoal, 
that the quantities of adsorbed anions and cations are 
not strictly equivalent to each other, that therefore 
an exchange adsorption has occurred to some extent, 
although in other cases charcoal is characterized by 
its apolar behaviour. Probably these two kinds of 
adsorption do not depend on one and the same inter- 
face. The exchange adsorption might well be caused 
by impurities in the charcoal (such as difficultly 
soluble phosphates) whilst the apolar adsorption 
would be due to the carbon itself. 

In spite of the differences between polar and apolar 
adsorption, we must not, however, immediately 
conclude that they are two essentially different pro- 
cesses. The adsorption isothermal applies also to 
the taking up of ions in exchange adsorption, and 
often the capillary-active ions are adsorbed more 
strongly than the capillary-inactive, which is indeed 
similar to the behaviour of non-electrolytes. 

The influence of temperature on the adsorption 
from solutions is generally slight. Mostly the adsorp- 
tion decreases with rise of temperature, although cases 
of an opposite behaviour are known. 



SOLID AND LIQUID 67 

We may now discuss a few specific examples. The 
well-known blue coloration of starch by iodine partakes 
so largely of the nature of an adsorption, that the 
majority of investigators regard it as such. Starch 
is not the only substance capable of forming with 
iodine a loose addition compound having a blue colour. 
There are a large number of organic, and also a few 
inorganic substances, which do the same. Among the 
former, xanthone and flavone derivatives may be 
specially mentioned. Thus a-napthoflavone, of the 
annexed constitution, is more sensitive to iodine than 




is starch. The behaviour of many of these substances 
is so peculiar and throws so much light on the nature 
of the adsorption process, that we may refer to a few 
details. The flavone derivatives in question are mostly 
very little soluble in water. In order to demonstrate 
their adsorptive power, their alcoholic solution is 
poured into an excess of an aqueous iodine solution. 
During the first few moments the alcoholic-aqueous 
solution may remain supersaturated and then no blue 
coloration appears. A deep blue coloration may then 
appear more or less suddenly, and it can be shown 
that the organic substance has separated in amorphous 



68 COLLOIDAL CHEMISTRY 

form (or in excessively minute crystals). In course 
of time (after several hours or days) the amorphous 
(or micro-crystalline) particles are transformed into 
larger crystals and then generally relinquish their 
iodine to the solution. We therefore end up with 
larger colourless crystals suspended in a solution of 
iodine. Hence the adsorptive power is much greater 
in the amorphous-solid condition. As long as the 
organic substance is in true solution, or after it has 
again separated in larger crystals, its power of adsorp- 
tion is slight or cannot be demonstrated. Some 
members of this group, the glucoside saponarin and 
euxanthic acid, form dilute true solutions in water, 
which are not coloured blue by iodine, and also more 
concentrated colloidal ones, which are coloured blue 
and lose their colour when enough water has been 
added to make them molecular-disperse. 

The adsorbent properties of paper have been in- 
vestigated in various ways. Paper shows almost 
exclusively an exchange adsorption, particularly with 
respect to dyes like methylene blue, the coloured 
cation of which is retained by the paper. If we let a 
drop of such a dye solution flow on to filter paper, 
we obtain a strongly coloured central patch, with a 
sharp border, surrounded by a colourless zone of pure 
water. The sharp outline of the coloured patch de- 
pends on the shape of the adsorption isothermal (see 
Fig. 9). The quantity adsorbed does not simply 
decrease in direct proportion to the falling concen- 
tration, but a very dilute solution still corresponds to 



SOLID AND LIQUID 69 

a considerable quantity of the adsorptive. If this 
quantity simply decreased in the same ratio as the 
concentration, we would have a very ill-defined bor- 
der. The falling concentration at the periphery of the 
spreading drop would then bring about the adsorption 
of smaller and smaller quantities of the dye. Actually, 
however, the absence of proportionality makes a very 
dilute solution compatible with considerable adsorp- 
tion. It is on this phenomenon that Goppelsroder has 
based an analytical procedure for the recognition and 
separation of substances, so-called capillary analysis. 
If a broad strip of filter paper be suspended with its 
lower edge in a solution of two dyes, which are not 
adsorbed with equal readiness, the more strongly ad- 
sorbable will be found immediately above the surface 
of the solution, while the other one will ascend higher. 
For instance, with a mixture of methylene blue and 
eosin we find below a blue zone containing a mixture 
of both dyes, and above a red one of eosin alone. 
The process is not confined to coloured substances ; 
colourless ones may be separated and recognized in 
the same manner, if the paper is afterwards treated 
with reagents which form coloured compounds with 
the substances to be separated. 

A loose union, reminiscent of exchange adsorption, 
is observed in certain silicates, the naturally occurring 
zeoliths and the artificial permutites. These, for 
example, take up Ca" ions from a solution, and give 
off Na'-ions ; the dependence on the concentration is 
similar to that shown by an adsorption isothermal. 



70 COLLOIDAL CHEMISTRY 

Under certain conditions a sodium permutite may 
give up almost the whole of its sodium in exchange 
for another ion, e.g. Ag'-ion. The silver has therefore 
permeated the whole of the permutite structure. Now 
these silicates are indeed extraordinarily porous and 
we cannot altogether reject the notion that the sili- 
cate ions may form an open and yet solid framework 
which allows the cations to permeate it thoroughly. 
For the present it must remain undecided, whether 
this conception is useful or whether the hypothesis of 
mixed crystals, hence of a so-called solid solution, will 
prove more fruitful. The soil has similar properties, 
i.e. it can bind cations loosely by exchange ; this is 
attributed to the presence of zeoliths and of humus, 
which can take up ions in a like manner. 

Adsorption is doubtless also important in a number 
of biological processes. Cell walls may adsorb dis- 
solved substances exactly as other adsorbents do. 
This has been proved for the fixation of corrosive 
sublimate by blood corpuscles and for that of phenol 
and other substances by yeast cells. Probably the 
first stage in disinfection by substances like corrosive 
sublimate is simply the adsorption of the antiseptic, 
which is only later followed by chemical changes. 
Among the many other intoxication processes, which 
closely resemble an adsorption, we may refer to the 
action of veratrine on the cardiac muscle of a marine 
snail (Aplysia limacina), investigated by Straub. All 
the characteristics of an adsorption are present : the 
veratrine is taken up rapidly and reversibly ; it can 



SOLID AND LIQUID 71 

therefore be removed again by washing, and the pro- 
cess may be represented by an ordinary adsorption 
isothermal. 

ADSORPTION CATALYSIS IN SOLUTION 

As we have seen, a reaction may be influenced by 
adsorption at the interface between a solid and a gas ; 
the same phenomenon may take place in solution and 
present the same diversity. 

A very clear illustration is furnished by the decom- 
position of hydrogen peroxide on the surface of glass 
wool, in the presence of metallic salts, such as those 
of copper. The reaction proceeds according to the 
equation 

2H 2 O a = 2H 2 + O a 

Glass wool by itself accelerates the reaction, so does 
a small quantity of copper salt, but if the copper salt 
and the glass wool act simultaneously, the rate of 
decomposition of the hydrogen peroxide is about ten 
times as great as when either agent acts singly. Now 
the copper is found to undergo adsorption on the glass 
wool, so that its concentration in the adsorption layer 
is much greater than in the solution, and consequently 
the catalytic acceleration, due to it, is also greater, 
since the reaction takes place almost exclusively on 
the surface. The rates of decomposition are indeed 
proportional to the quantities of copper adsorbed. 

A close analogy to the decomposition of stibine, 
described on p. 50, is provided by the decomposition 



72 COLLOIDAL CHEMISTRY 

in solution of formic acid, according to the equation 

HCOOH - H, + CO a 

which decomposition can be accelerated by finely 
divided rhodium and particularly by osmium. The 
analogy extends to the course of the reaction and to 
its dependence on the concentration. Here also the 
two products of the reaction (H 2 and CO 2 ) are diffi- 
cultly adsorbable, in comparison with the formic acid, 
and the reaction proceeds as if only those molecules 
of the acid are decomposed, which undergo adsorption. 
The retardation of a reaction in the adsorption 
layer by other foreign substances was mentioned in 
reference to gaseous adsorption ; it is also found in 
solution. Thus charcoal accelerates the oxidation of 
phenylthiocarbamide by free oxygen to sulphur and 
an unknown sulphur derivative of phenylthiocarba- 
mide, and this reaction is definitely retarded by the 
products of the reaction. The oxidation of oxalic 
acid, according to the equation 

2C 2 H 2 O 4 + O 2 = 4CO 2 + 2H 2 O, 

is also accelerated by charcoal, and we have here the 
curious case, that a reacting substance (oxalic acid) 
retards its own oxidation. The reaction proceeds the 
more slowly, the more oxalic acid is present in the 
adsorption layer, doubtless because the oxygen has to 
diffuse through this layer and does so all the more 
slowly, the thicker (or denser) the layer is. Foreign 
organic substances, such as the urethanes, also retard 
the reaction, and, as might be expected, all the known 



SOLID AND LIQUID 73 

characteristics of adsorption appear. The adsorba- 
bility of the urethanes is known to increase rapidly, 
as we ascend the homologous series, and to follow 
Traube's rule. Accordingly ethyl ure thane retards 
several times as strongly as methyl urethane, and 
propyl urethane excels the ethyl compound in the 
same ratio. The retardation is doubtless due to the 
fact that these foreign substances make the access of 
oxygen even more difficult. 

A still deeper insight into the mechanism of such a 
reaction is afforded by the investigation of the oxida- 
tion in aqueous solution of amino-acids, such as cystine, 
on the surface of charcoal. While neither the original 
substance nor the reaction products strongly retards 
the reaction, a number of foreign substances do so 
powerfully. Among these substances two groups may 
be distinguished. There are in the first place capillary- 
active organic substances, which act quite like the 
urethanes in the previous example. Their retarding 
action is therefore found to increase like their adsorb- 
ability, in accordance with Traube's rule. The action 
is probably due to these substances displacing the less 
adsorbable amino-acid from the surface and thus lessen- 
ing its surface concentration and thereby also the 
velocity of its oxidation. 

There is, however, a second group of substances 
which retard the reaction about 1,000 times as strongly 
as the organic substances and yet are not appreciably 
capillary-active, nor are they strongly adsorbed ; 
hydrocyanic acid is a striking example of this second 



74 COLLOIDAL CHEMISTRY 

group. Otto Warburg explains its peculiar action as 
follows : By no means the whole surface of the char- 
coal takes part in the reaction, but only certain por- 
tions are active, where iron compounds (or compounds 
of other heavy metals) occur. The importance of iron 
results from the fact, that all charcoals containing 
iron accelerate the oxidation of the amino-acids, whilst 
varieties completely free from iron are not active at 
all, but become so when a minute quantity of iron 
has been incorporated in them in a suitable form. 
The hydrocyanic acid is considered to be loosely 
bound, especially by the iron compounds, which thus 
lose their catalytic activity. The interpretation of 
the behaviour of capillary-active substances, given 
above, remains unaffected, for when the latter are 
adsorbed by the charcoal, they cover the whole of the 
charcoal surface, including the ferrous portions, and 
displace the amino-acid from them. Charcoal and 
iron are therefore in the same relation as are glass wool 
and copper in the previously discussed example of the 
decomposition of hydrogen peroxide. 

These reactions taking place at interfaces are of 
great biological importance. 0. Warburg has shown 
that the reactions underlying respiration, fermentation 
and carbon assimilation also take place at surfaces. 
The proof was partly furnished by the observation 
that all these processes are retarded by capillary- 
active substances, which follow Traube's rule. For it 
is practically certain that these influences only accom- 
pany interfacial reactions, and not reactions taking 



CAPILLARY-ELECTRICAL PHENOMENA 75 

place in true solution. Ferment reactions are doubt- 
less of the same nature, whether the ferment be pre- 
sent in colloidal solution, or whether it be adsorbed 
at an interface. Many ferment actions, e.g. the 
decomposition of hydrogen peroxide by catalase, have 1 
been found to resemble clearly other reactions which 
are accelerated by particles in colloidal solution, e.g. 
the decomposition of hydrogen peroxide by colloidal 
platinum. This resemblance may be partly explained 
by the circumstance that interfacial processes are 
important in both cases. 

5. Capillary-electrical Phenomena 

Electrical influences are of considerable importance 
in the study of colloids, but are here of a quite different 
kind from those with which electrochemistry has hitherto 
chiefly concerned itself. We have to consider here the 
so-called electrokinetic processes, which do not appear 
at all in galvanic cells, and only slightly in electrolysis. 
These processes may be readily illustrated by the fol- 
lowing experiment : Milk is poured into a U-tube, as 
shown in Fig. n, and, without mixing, a layer of pure 
water is introduced above the milk into both limbs. 
Platinum electrodes, E and E', attached to wires, dip 
into the water layers. When a potential difference 
of 100 Volts is set up between the electrodes, the bor- 
der surface separating the milk from the water moves 
away from the negative electrode in one limb, and 
towards the positive electrode in the other. The fat 



7 6 



COLLOIDAL CHEMISTRY 



globules of the milk therefore migrate with a certain 
velocity, as the result of the potential difference applied 
from outside, and they behave as if negatively charged. 
This phenomenon is called cataphoresis and we speak 
of the cataphoretic migration velocity 
of the globules. In this example 
the fat globules happen to be 
liquid, but solid particles, liquid 
droplets and gas bubbles all behave 
in the same way ; the state of 
E. aggregation of the particles is a 
matter of indifference. 

In the cataphoresis of solid par- 
ticles the rigid interface is mobile, 
the liquid as a whole does not 
move. The relationship is reversed 
by the arrangement represented in 
Fig. 12. Dis a porous diaphragm, 
e.g. an asbestos plug or a gelatin 
jelly. On either side are electrodes 
and the vessel K is quite full of 
water, which rises to a certain 
height in the capillary R. If a 
current of about 100 volts or more 
is passed through, the water moves, as if it were 
positively charged, i.e. it rises higher in the capillary 
R. This (inverted cataphoresis) is electrosmosis ; it 
will be seen that the walls of the diaphragm pores are 
fixed, that the liquid can move and that it migrates 
as the result of an external potential difference. 




FIG. ii. 



CAPILLARY-ELECTRICAL PHENOMENA 77 

Whilst in this respect electrosmosis is the reverse 
of cataphoresis, both phenomena may be reversed in 
another way. If in the apparatus in Fig. 12 no 
potential gradient be applied, but the liquid be 
forced through the diaphragm by means of a pump, 
an electrometer will indicate a potential difference 
between the two sides of the diaphragm. Water 
is charged positively with respect to the material 
of most diaphragms, and accordingly the electrode, 






FIG. 12. 

towards which the water is pumped, is charged posi- 
tively in relation to the other. The potentiaUset up 
in this manner is called a stream potential. 

The corresponding inversion of cataphoresis is 
achieved, for instance, when a powder falls through 
a liquid between two electrodes ; the potential differ- 
ence set up between the latter can then be detected. 
This is the potential due to falling particles. 

The following table summarizes the relationship 
between the four phenomena : 



78 COLLOIDAL CHEMISTRY 

I. A potential difference applied from outside pro- 
duces a movement : 

(a) of the liquid relative to a fixed interface : 

electrosmosis. 

(b) of solid particles relative to the liquid : cata- 

phoresis. 

II. A movement due to external forces produces a 
potential difference : 

(a) The liquid is made to move relatively to the 

fixe,d interface : stream potential. 

(b) The solid particles are made to move relatively 

to the liquid : potential due to falling particles. 

A theory of the purely physical aspects of electro- 
kinetic phenomena was developed long ago by Helm- 
holtz. It deals, for instance, with the way in which 
the cataphoretic migration velocity depends on the 
size of the particles, on the viscosity of the liquid, 
on the external difference of potential, etc. The theory 
has been abundantly confirmed ; in particular the 
four above-mentioned phenomena are actually found 
to be connected quantitatively in the manner required 
by the description given. Any result obtained, for 
example, with electrosmosis, may therefore at once 
be applied to cataphoresis or to stream potentials. 

In Helmholtz's theory an important part is played 
by the potential difference between the fixed wall and 
the liquid. In the following discussion this quantity 
will be termed the electrokinetic potential difference. 
Helmholtz did not further consider the properties of 
this potential difference, its dependence on dissolved 



CAPILLARY-ELECTRICAL PHENOMENA 79 

substances, etc. The question arose, however, when 
Nernst's theory of galvanic cells also postulated a 
potential difference at every single electrode ; the 
dependence of this potential on dissolved substances 
is deducible from Nernst's theory with great accuracy. 

We might at first be inclined to assume, that the 
electrokinetic potential difference and Nernst's poten- 
tial difference at a single electrode of the galvanic 
cell, which latter potential we will call e, are identical. 
This view was often put forward. 

Facts were soon discovered, however, which mili- 
tated against the supposed identity, and when the 
quantities and e were then measured at interfaces, 
as far as possible similar, they turned out to be largely 
independent of each other. Nernst's potential is 
mostly determined at metallic electrodes of galvanic 
cells. We might therefore have attempted, for pur- 
poses of comparison, to measure the electrokinetic 
potential at metallic surfaces, e.g. in the cataphoresis 
of metallic particles or in electrosmosis through metal- 
lic diaphragms. Now Helmholtz's theory of electro- 
kinetic phenomena applies in the first place only to 
bad conductors ; it is not unconditionally applicable 
to metals. It was therefore desirable to measure the 
potential at the surface of a bad conductor, such as 
glass, at which Nernst's potential e (often called the 
phase-limit force) may also be determined. This can 
be done by means of the apparatus represented in 
Fig. 13. A is a very thin glass bulb (having a wall 
less than 0*1 mm. thick) filled with an electrolyte 



8o 



COLLOIDAL CHEMISTRY 



solution which does not undergo any change, and 
into it dips a platinum wire, serving as electrode. 
The bulb A is immersed in a beaker B containing the 
solution to be investigated. E is a normal electrode, 
such as is also used in other cases as a standard of refer- 
ence. (In a galvanic cell we never measure the poten- 
tial of a single electrode, but always the sum or dif- 
ference of at least 
two potentials. 
Thus if we desire 
to compare the 
potentials of a 
metal in two 
different solutions, 
the other electrode 
of the galvanic 
cell must remain 
unaltered ; it is 
the standard of 
reference.) If we 
connect the nor- 
FIG. 13. mal electrode with 

that inside the 

glass bulb, we measure potential differences, which 
vary in a quite regular manner when the electrolyte 
solution in B is varied. The conductivity of the glass 
is sufficiently large, and the wall of the bulb sufficiently 
thin, for electricity to pass through the bulb and the 
platinum wire. The glass bulb therefore behaves like 
any metallic electrode of a galvanic cell, and thus 




CAPILLARY-ELECTRICAL PHENOMENA 81 

the phase-limit force s of glass against an aqueous 
solution may be determined. 

For purposes of comparison the solutions used in 
the above experiments were forced through capillaries 
made of the same kind of glass as the bulb ; the 
resulting stream potentials, and with them the electro- 
kinetic potential differences f of the capillary wall, 
were then determined. It was found that e and f 
behave quite differently. In the experiments with 
the glass bulb only acids and bases had a definite 
effect. Aluminium salts and salts with a strongly ad- 
sorbable cation, such as dyes (crystal violet), hardly 
affected the phase-limit force e. In the case of the 
stream potentials the effect of acids and bases was 
evident, but small compared with that of aluminium 
salts and dyes ; with crystal violet, for instance, the 
charge on the glass capillary, used for measuring stream 
potentials, could actually be reversed. Originally 
negative with respect to pure water, this charge 
became positive in the presence of the dye. 

This behaviour may be interpreted by assuming for 
the potential at a fixed interface a course represented in 
Fig. 14. We will suppose that the fixed wall is on the 
left of A, and the liquid on the right ; between A and 
B is the portion of the liquid which firmly adheres to 
the wall. The abscissae x, plotted from A to the right, 
are the distances from the wall ; the ordinates are 
the potentials at the corresponding distances x. The 
full curves i and 2 represent two possible courses of 
the potential e at the wall. We see that we must 

6 



82 



COLLOIDAL CHEMISTRY 



distinguish two potential differences ; in the first place 
/, representing the difference between the poten- 
tial e of the fixed wall itself and the potential s f in 
the interior of the fluid this difference is Nernst's 




_*z * 

FIG. 14. 

potential or the phase-limit force ; in the second place 
we have e h f, the difference between the potential 
e h of the layer of liquid adhering to the wall, and the 
potential e f of the interior of the fluid this difference 
is the electrokinetic potential . The dotted line pass- 



CAPILLARY-ELECTRICAL PHENOMENA 83 

ing through B represents the boundary between the 
layer of fluid adhering to the wall and the interior of 
the liquid ; since the curves I and 2, representing the 
course of the potential, intersect this dotted line in two 
different places, there are two different values for e h 
and therefore also for . With respect to e it is evid- 
ently important that ions can pass from the interior 
of the liquid into the wall itself. If, as usual, the 
electrode consists of a 'metal, then, as is well known, 
the ions of this metal are the determining factors, 
according to Nernst's theory ; in the case of a zinc 
electrode zinc ions pass from it into solution, or dis- 
solved zinc ions are deposited on the electrode as 
metallic zinc. The fact that with a glass electrode the 
-potential is especially altered by acids and alkalies, 
is doubtless due to the power of H'- and OH'-ions to 
alter the silicate framework itself ; f on the other hand 
will be influenced by any ion whatsoever which is 
present at the interface, especially in consequence of 
adsorption. We also see from Fig. 14, how for identical 
values of s the potential may be very different, lor 
in the two cases represented f has an opposite sign for 
identical values of e ; this is because in case 2, as a 
result of the presence of different ions, not occurring 
in case i, and of their different adsorbability, the course 
of the potential is a different one. 

The above hypothesis therefore explains why the 
electrokinetic potential is so largely independent of 
Nernst's potential, and also that the former, unlike the 
latter, is greatly influenced by the adsorbability of 



84 COLLOIDAL CHEMISTRY 

the ions. Unfortunately we do not know enough 
about the adsorption of individual ions and the way 
in which they crowd each other out. We cannot 
therefore as yet obtain any idea of the actual course 
of the curves i and 2 in any particular case. We 
merely call attention to a few general results, which 
will be applied later in our discussion of colloidal 
solutions. In investigating the effect of increasing 
concentrations of an electrolyte on the f potential 
of a layer adhering to a wall, it is, as has been 
pointed out, immaterial whether we employ cata- 
phoresis, or electrosmosis, or stream potentials for 
this purpose. Whatever the method employed, we 
find the most effective ion to be the one, which carries 
a charge opposite to that of the wall layer. In Fig. 15 
the abscissae represent electrolyte concentrations in 
milligram-molecular weights (millimols) per litre, the 
ordinates are t-potentials, determined from the cata- 
phoretic migration velocities of petroleum droplets in 
the respective electrolyte solutions. 

The droplets or what matters in cataphoresis, their 
surface layers are negatively charged. Hence the 
f -potential in Fig. 15 has been plotted upwards with 
negative and downwards with positive sign. We now 
see how in the case of the univalent K'-ion, the anion 
has a distinct effect : the C-potential is increased, the 
charge on the droplets is raised to some extent ; the 
effect is smallest with the univalent Cl'-ion, largest with 
the quadrivalent Fe(CN) 6 ""-ion. But with cations of 
higher valency, the bivalent Ba"-ion, the trivalent 



CAPILLARY-ELECTRICAL PHENOMENA 85 

AT'-ion and the quadrivalent Th""-ion, the effect of 
the anion becomes less prominent owing to a decrease 
of the f -potential ; there is a lowering of the charge 



008 



006 



A 

-004 



002 



002 



004 



ooe 




AlCL 



FIG. 15. 

and the discharging effect increases quite strikingly with 
the valency of the cation. 

We must add, that valency is not the only factor 
determining this phenomenon, If we compare the 
K'-ion with organic cations, e.g. with that of aniline 



86 COLLOIDAL CHEMISTRY 

or of a dye such as crystal violet, we find that the 
organic cations lower the f -potential much more 
strongly, and therefore have a greater discharging 
action, than the K'-ion or other univalent inorganic 
cations. The curve for aniline corresponds more or 
less to that for Ba"-ions, that for crystal violet roughly 
to the Th""-ion curve in Fig. 15. This stronger action 
of organic cations is connected with their greater 
adsorbability. 

If on the other hand the layer on the wall is charged 
positively from the beginning, instead of negatively, 
it is the effect of the anions which preponderates in 
much the same way. With them also it is a question 
of valency and adsorbability. 

The electrokinetic potential is therefore primarily 
affected by those ions, which have a charge opposite to 
that of the wall layer, and they are the more effective 
in lowering this potential, the higher is their valency 
and the greater their tendency to be adsorbed. 

It is beyond the scope of this book to discuss the 
numerous phenomena connected with electrokinetic 
processes, although many are of technical or biological 
importance. We will only allude to a few examples. 
Electrokinetic influences appear in osmosis. The 
osmosis of solutions of non-electrolytes is the experi- 
mental starting-point of the modern theory of solutions, 
for the fundamental example is the osmotic penetra- 
tion of water into a cane-sugar solution. The latter is 
contained in an unglazed earthenware pot, with pores 
blocked by copper ferrocyanide, a type of cell devised 



CAPILLARY-ELECTRICAL PHENOMENA 87 

by Pfeffer, This osmosis may be satisfactorily inter- 
preted by means of the osmotic pressure of the sugar 
solution. The phenomenon becomes much more com- 
plicated, if we use electrolytes instead of sugar, and, 
instead of copper f errocyanide, membranes of collodion, 
with or without gelatin. 

Unlike Pfeffer's cell, these membranes are not strictly 
semi-permeable ; in addition to water, they also allow 
salt to pass. The departure from a regular osmotic 
behaviour may be of various kinds. With many elec- 
trolytes we observe an extraordinary osmosis, which 
falls off at greater concentrations and begins to increase 
only at still greater ones as required by the augmented 
osmotic pressure of the solution. Indeed, electrolytes 
are known, which from the outset show a negative 
osmosis, that is to say, water does not penetrate into 
the salt solution, but the salt solution passes out into 
the water. Recent experiments by Jacques Loeb have 
definitely established that this behaviour depends on 
the intervention of an electrosmosis. A potential 
difference is set up between the two sides of the mem- 
brane, the [so-called membrane potential, and like the 
external potential difference in electrosmosis, this drives 
the liquid through the pores of the membrane, provided 
certain conditions are fulfilled, which allow of the 
generation of an electric current. Depending on the 
algebraic sign of the membrane potential, water is 
driven into the salt solution (osmosis) or salt solution 
into the water (negative osmosis). Since this varying 
transport is superimposed on the movement of water 



88 COLLOIDAL CHEMISTRY 

due to osmotic pressure, the nett result should show 
great variety, which is indeed actually observed. 
Very probably such negative osmosis is important in 
glandular secretion, in the bleeding of plants, etc. 

Technical applications of electrosmosis have been 
attempted, with some measure of success, by Count 
Schwerin and the " Elektro-Osmose " company founded 
by him. Thus peat may be dried by subjecting it 
between perforated electrodes to an electric current, 
which drives out the water by electrosmosis, so that 
it drips out through the holes in the electrode. Again, 
in order to obtain a high-grade clay, an impure variety 
is suspended in water, and after allowing the coarser 
particles to settle, the specially plastic finest particles 
and the impurities are precipitated by cataphoresis. 
In electrosmotic tanning the hides are hung between 
perforated electrodes ; not only is washing accelerated 
by electrosmosis, but owing to cataphoresis, the tannin 
also penetrates more rapidly into the hide. 

6. The Properties of Interfacial Layers 

Hitherto we have always discussed adsorption layers 
without considering their thickness, or the question 
whether any special behaviour should be attributed to 
the molecules contained in them. Our present know- 
ledge on these points is slight and not well-founded, 
but slight though it be, it is important for an under- 
standing of surface processes. We may begin by 
enquiring whether the adsorption layer consists of a 



INTERFACIAL LAYERS 89 

single layer of molecules or whether it comprises several 
such layers. Many investigators are of opinion that 
generally, if not always, the layer is one molecule thick 
and that it need not even be continuous, but that the 
molecules of the adsorbed substance may be distributed 
at various points, with intervals between them, some- 
what as if on a chessboard the black squares only might 
be occupied by the pieces. A continuous layer might 
have a thickness of I ///* or even less, according to the 
diameter of the constituent molecules. In contradis- 
tinction to this, other investigators believe that the 
layer consists of two or three, or even of ten to twenty 
layers of molecules, so that it would have to be 5-10 w 
in thickness. 

We cannot here consider the respective merits of 
these alternative hypotheses and we merely point out 
that in electrokinetic phenomena more especially, it is 
difficult to conceive of the adsorption layer as consisting 
of a single layer, which does not extend into the interior 
at a greater distance from a solid wall than one mole- 
cular diameter. (See Fig. 14.) The adsorption of 
gases by charcoal and its dependence on temperature 
may moreover be calculated on simple assumptions, 
if we suppose the adsorption layer to be several mole- 
cules in thickness. 

On the other hand it seems pretty definitely estab- 
lished that there are examples of adsorption layers, or 
quite generally of fluid membranes, which consist of 
only one layer of molecules. Since these allow various 
interesting conclusions to be drawn, we will deal with 



go COLLOIDAL CHEMISTRY 

them a little more fully. On p. 36 we discussed the 
behaviour of two immiscible liquids, and in particular 
the case in which one liquid spreads completely over the 
surface of another. If therefore a suitable liquid, e.g. 
oleic acid, is placed on a clean water surface, it is drawn 
out to a thin film which covers the water. Now there 
is a maximum surface, and therefore also a minimum 
thickness, to which the oily film can be drawn out ; if 
we try to exceed this maximum surface, the film breaks 
away. Devaux has carried out measurements of this 
kind with very simple means. He filled a rectangular 
photographic dish with water and carefully cleaned 
the surface of the liquid by drawing a strip of paper 
across it so as to collect the impurities at one end. 
Behind this strip, which remained to some extent as a 
line of demarcation, a clean water surface was formed, 
on which a minute quantity of oil was placed. The 
oil spread over the surface, but not over the whole of it, 
if the quantity of oil was sufficiently minute. The area 
covered by the oil may be recognized by cautiously 
dusting with talcum powder. Since the contour of 
the oil film is not sufficiently regular for purposes 
of measurement, a second strip of paper is advanced 
from the side of the clean surface, so that the oily film 
is confined to a rectangular area between the two strips 
of paper. If the second strip be moved backwards, the 
edge of the oil film, recognizable by the talcum dust, 
detaches itself from the paper, when the latter is in a 
given position. By moving the paper to and fro several 
times, this limiting position can be exactly determined, 



INTERFACIAL LAYERS 91 

and with it the maximum surface which the oil can 
cover. From the weight of oil used and its density, 
the thickness of the film can be calculated. 

In this way the thickness of a film of triolein was 
found to be about I pp ; this is about the thickness of 
a single layer of molecules ; a substance of high mole- 
cular weight, like triolein, has of course molecules of 
considerable size. 

As Langmuir and Harkins have shown, experiments 
of this kind allow far-reaching conclusions to be drawn 
with regard to the shape of the molecules. Such 
speculations may perhaps seem ill-founded, but there 
is an ever-increasing body of evidence, suggesting that 
molecules have a definite shape, about which the above 
considerations may enlighten us. We have already 
referred in the introduction (p. 3) to the number of 
molecules present in a given quantity of a substance. 
A grammolecular weight (885 grams) of triolein con- 
tains 0-6 Quadrillions of molecules. Since the weight of 
the triolein placed on the surface is known, the number 
of molecules can be calculated. Now if the oil film 
is only one molecule thick and the molecules lie side by 
side, we obtain the area occupied by each individual 
when we divide the maximum surface of the film by the 
number of molecules in it. For triolein this area works 
out at I 3 trillionth sq. cm. Quite a number of organic 
substances have been examined in this way, and a 
comparison of the results shows the molecules of such 
different substances as palmitic acid Ci B H 8 i.COOH, 
stearic acid C n H M .COOH, cerotic acid C W H M .COOH 



9 2 COLLOIDAL CHEMISTRY 

and myricyl alcohol C 30 H 61 OH to occupy practically 
the same area in the film. A molecule of tristearin 
CsH^CisHasOa^ occupies almost exactly three times 
the area of a molecule of stearic acid, and one of tri- 
olein three times that of oleic acid. 

Now this behaviour may be explained as follows : 
All the substances referred to are polar in character ; 
at one end they have a group, like -COOH, -OH, or 
(-C^sCaHg, which is markedly hydrophilic and thus 
has a great affinity for water ; the other end is occupied 
by alkyl groups, which are hydrophobic and have little 
affinity for water. It seems plausible to assume, that 
the affinity for water will affect the orientation of the 
molecules, in such a way that the hydrophilic groups 
are turned towards the water and the hydrophobic ones 
away from it. Now palmitic, stearic and cerotic acids 
as well as myricyl alcohol, all have the same hydro- 
phobic group pointing upwards, namely a methyl group, 
and thus the area occupied by these groups is also the 
same. These polar molecules are to some extent com- 
parable to rods with dissimilar ends, such as pointed 
lead pencils. They all lie close together in the oily 
layer and all point a similar end upwards. It is prob- 
ably more correct to assume, as indicated by other 
experiments, that the long alkyl chains are folded like 
a concertina. In tristearin three rodlets are united 
to a single molecule and three methyl groups point 
upwards. The area occupied by them is three times 
as large as that occupied by the single methyl group 
of stearic acid, The same ratio holds good between 



INTERFACIAL LAYERS 93 

triolein and oleic acid, only the area occupied by the 
latter is larger than that occupied by stearic acid. 

We may mention some further conclusions which 
also seem to be well-founded. It will be remembered, 
that the lower fatty acids, which are soluble in water, 
are strongly capillary-active ; in accordance with 
Traube's rule successive members of the homologous 
series depress the surface tension more than their next 
lower homologues and are adsorbed on the surface with 
increasing readiness. (See pp. 29 and 62.) In these 
dilute solutions the molecules of these fatty acids are 
probably not arranged at right angles to the surface, 
as in the oil films discussed above, but probably lie 
horizontally with their whole length in the surface. 
It is found that this assumption is quite sufficient to 
explain the regular increase in capillary activity 
according to Traube's rule. In cases where this rule 
holds good, we may therefore assume that the entire 
molecule lies in the adsorption layer. 

Considerations of this kind are also important in 
connexion with catalysis in an adsorption layer. A 
reaction between two molecules with polar ends will be 
accelerated, if the ends, which are required to react, 
come into contact in the surface. But if this is not the 
case, if a hydrophobic end is required to react with a 
hydrophilic one, and the hydrophobic ends of the mole- 
cules are directed towards the adsorbent, while the 
hydrophilic ones point to the water, the reaction is not 
accelerated by the adsorbent, and may even be retarded 
by it. 



94 COLLOIDAL CHEMISTRY 

B. THE RATE OF FORMATION OF A NEW 
PHASE 

Many colloidal solutions are prepared by subjecting 
a dissolved substance to a chemical reaction which 
results in the separation of a solid. Thus colloidal gold 
solutions, to which we shall often refer later, are pre- 
pared by reducing a gold chloride solution containing 
potassium carbonate, by means of formaldehyde, In 
this reaction gold first separates as atoms, which unite 
to form gold nuclei, and these may then grow to gold 
crystals. In any case therefore we must distinguish two 
rates : the rate of reaction which leads to the forma- 
tion of gold nuclei, the rate of nuclear formation, that is 
the rate at which gold atoms unite to form the smallest 
gold crystals, and the rate of crystallization, at which 
the gold nuclei grow to larger crystals. Now the rate 
of nuclear formation and the rate of crystallization have 
been studied in systems having no direct connexion 
with colloids. It is therefore convenient to discuss 
these systems first. A similar consideration applies to 
the nature of the amorphous-solid state. As was 
pointed out in the introduction, this state is not charac- 
teristic of the colloidal condition as such, but since 
colloidal particles are often amorphous, an understand- 
ing of amorphous solids is desirable for the study of 
colloids. 

RATES OF NUCLEAR FORMATION AND OF 
CRYSTALLIZATION. 

We must first consider the origin of nuclei and their 
growth in the undercooled melts of pure substances. 



FORMATION OF A NEW PHASE 95 

These processes do not themselves form part of the 
subject of colloids, but will serve to illustrate the 
essential features of the phenomena involved. 

Many liquids can readily be cooled below their freez- 
ing-point without solidifying ; they are then under- 
cooled. If a small crystal of the frozen liquid be then 
introduced, the whole of the liquid crystallizes from 
this point. Glazed frost is a phenomenon depending 
on undercooling. The soil, the pavement, etc., are 
covered with a film of water, which at first remains 
undercooled when the temperature falls, and then sud- 
denly crystallizes to a thin coating of ice, as soon as 
ice nuclei appear. In the case of water the origin of 
crystal nuclei and the subsequent crystallization cannot 
be followed very closely, since both processes take place 
too rapidly, but in a number of organic liquids which 
freeze at suitable temperatures, these processes proceed 
sufficiently slowly for convenient study. 

We can readily observe in the case of pipeline, fused 
between two glass plates, how on cooling below the 
melting point, crystal nuclei are formed spontaneously 
in various places, from which the crystallization pro- 
ceeds. If we lower the temperature still further, the 
nuclei at first become more numerous. The number of 
nuclei formed in unit time represents the rate of nuclear 
formation, and this rate at first increases with falling 
temperature. Generally it is difficult to determine at 
low temperatures, because the nuclei grow too slowly. 
Here Tammann, the principal student of these pheno- 
mena,*uses the following artifice. He keeps the melt 



9 6 



COLLOIDAL CHEMISTRY 



for some time at the temperature at which he desires 
to measure the rate of nuclear formation, and then 
raises it to a higher temperature, at which the rate of 
crystallization is large enough for the nuclei, formed at 
the lower temperature, to grow rapidly into visible 
crystals. In this way the rate of nuclear formation 

was found to increase at 
first with falling temper- 
ature, and then to de- 
crease again. Fig. 16 
shows the way in which 
the rate of nuclear for- 
mation depends on the 
temperature in the case 
of undercooled piperine. 
Since this substance melts 
at 128, we see that the 
rate of nuclear forma- 
_ _ _ fle ^. Qn can Q , ^ studied 

Temperature * , . 

FlG I6 much below the melting 

point. 

It is indeed doubtful whether the phenomena, thus 
observed, are caused exclusively by the molecules of 
the undercooled liquid. The rate of nuclear formation 
is found to be greatly influenced both by dissolved 
foreign substances, and by undissolved coarser par- 
ticles ; in the case of many liquids there is good reason 
for believing that the observed rates of nuclear forma- 
tion primarily depend on the amount and the proper- 
ties of the dust contaminating the melt. The molecules 




FORMATION OF A NEW PHASE 97 

of the melt are adsorbed qn the surface of these dust 
particles, and are probably orientated in the manner 
described on p. 92 ; they thus unite to form a nucleus, 
provided that the number of the adsorbed molecules 
is large enough. For this reason caution is necessary 
in interpreting the course depicted in Fig. 16. The 
diminished rate of nuclear formation at low tempera- 
tures may probably be attributed to the increased 
viscosity of the melt, which is unfavourable to the 
union of molecules to nuclei. The increased rate at a 
somewhat higher temperature is doubtless due to the 
following : Not all the molecules of the melt are in a 
condition suitable for nuclear formation ; if this were 
so, every melt would crystallize at once below the 
freezing point. Only a fraction of the molecules are 
capable of this, namely those whose heat motion 
is below the average. Such slowly moving molecules 
can more readily cohere and form nuclei. If the tem- 
perature falls, the heat motion as a whole becomes less 
and so the fraction of slowly moving molecules, capable 
of nuclear formation, increases. At still lower tem- 
peratures this advantage is wiped out by the increased 
viscosity. 

With respect to crystallization, supersaturated solu- 
tions behave in much the same way as undercooled 
melts. Such solutions are obtained by dissolving the 
solute at a higher temperature, at which it is more 
soluble, and then cooling the liquid ; as a rule crystal- 
lization does not set in at once, but the solution remains 
supersaturated. The supersaturation may be increased 

7 



98 SKETCH OF THE HISTORY OF 

had some squadrons with him for the protection of 
that district, threw himself into a fortified house 
outside the Cabul Gate of the city. The Mahrattas 
surrounded him, and the next day he formed one of 
those desperate resolutions which have so often been 
known to influence the course of native politics. 
Putting on all his armour,* and wearing over it a 
sort of shroud of green, in the fashion used for the 
grave-clothes of a descendant of the Prophet, Nujuf 
Khan rode out at the head of his personal guards. 
As the small band approached the Mahratta camp, 
shouting their religious war-cries of "Allah Ho 
Ultbur" and " Ya Hossein," they were met by a 
peaceful deputation of the unbelievers who cour- 
teously saluted them, and conducted to camp in 
friendly guise. 

The fact was that the news of thePeshwa's death, 
which had recently arrived from Poonah, and the 
unsettled state of the Kohilla quarrel, combined to 
render the Mahrattas indisposed to push matters to 
extremity against a man of Nujuf Khan's character 
and influence, and thus gave rise to this extraor- 
dinary scene. The result was, that the ex-minister's 
excitement was calmed, and he agreed to join the 
Mahrattas in an attack on Rohilkund. One cannot 
but remark the tortuous policy of these restless 
rievers. First, they move the Emperor upon the 

* The armour of a Moghul noble consisted of a skull-cap and 
panoply of chain-mail, so exquisitely wrought of pure steel rings 
that the whole scarcely weighed ten pounds : over this he wore a 
morion T and four plates of steel, called char Aeen. 



THE MOGIIUL EMP1RB]. 99 

Rohillas ; then they move the Rohilla, Zabita Khan, 
upon the Emperor ; and then, having united these 
enemies, they make use of a fresh instrument to 
renew the original attack. With this new ally they 
marched upon Rohilkund by way of Hamghat, below 
Unoopshuhur, where the Ganges is fordable during 
the winter months. 

Meanwhile the British, finding that the Emperor 
was unable to protect the provinces which they had 
put into his charge, made them over to the Vice- 
roy of Oudh, to whose charge they had been attached 
previous to the negotiations that followed the battle 
of Buxar, and between whose dominions and those 
of the British they formed the connecting link. 
They had been abandoned by the Emperor when he 
proceeded to Dehli, contrary to 'the remonstrance of 
the Bengal Council, arid his own lieutenant had 
reported, and with perfect accuracy, that he could 
not regard the order to give them up to the 
Mahrattas as a free act of his master's. It would 
indeed, have been an easy step towards the ruin of 
the British to have allowed the Mahrattas to take 
possession of them. Yet this perfectly legitimate act 
of self-defence is thus characterized by Macaulay :* 
" The provinces which had been torti from the 
Moghul were made over to the government of Oudh 
for about half a million sterling." The British then 
joined their forces to those of the Vuzeer Viceroy 
Shoojaa, and marched to meet the invaders. The 

* " Critical and Historical Essays," art. " Warren Hastings," 



100 SKETCH OF THE UISTOltY OF 

Protector, whom we have lately seen treating with 
those powers, now became anxious about the money- 
payments for which he had engaged, in the usual 
reckless Oriental way, and entered into negotiations 
with the Malirattas.* In this scheme, the sudden 
arrival of the British and Oudh armies surprised 
him, and he was forced to abandon it for the present 
and join the allies in an advance against the Mah- 
rattas, who precipitately retired on Etawa, and thence 
to their own country, in May, 1773. 

Meerza Nujuf Khan was a family connection of 
Shoojaa-ood-Dowla, and an old friend of the British 
general ; and, on the retreat of his Mahratta sup- 
porters, he came over to the allied camp, where he 
met the reception due to his merits. 

The allied armies moved on to Unoopshuhur, 
accompanied by the ex-minister, who was attended 
by his faithful Moghuls. This town, which had, as 
we have seen, been a cantonment of Ahmud the 
Abdalee, was particularly well situated for the ad- 
vanced post of a power like the British, seeking to 
hold the balance among the native states of Hindoo- 
stan. To the north were the fords of Sookhurtal, 
by which the ]^ujeebabad Hohillas passed from one 
part of their dominions to another ; to the south 
was the ford of Ramghat, leading from Aleegurh to 
Bareillee. It remained a British cantonment from 
this time t until some time subsequent to the occu- 

* Hamilton's ''History of the liohilla Afghans." 
t With one or two hort interruptions, such aa during the 
brief ascendency of Fnmcis's opposition in the Calcutta Council. 



THE MOGHTJL EMPIRE. 101 

pation of the country in general, in 1806, after which 
the town of Meerut became more central, and 
Unoopshuhur ceased to be a station for troops. It 
is a thriving commercial entrepot in our days, though 
much menaced by the Ganges, on whose right bank 
it stands. The only memorial of the long-continued 
presence of a British force is now to be found in two 
cemeteries, containing numbers of graves, from which 
the inscriptions have disappeared. 

At this station Nujuf Khan took leave of his 
patrons, having received from Shoojaa-ood-Dowla 
the portfolio (or, to use the Eastern phrase, pen- 
case) of Deputy- Vuzeer, and from the British gene- 
ral a warm letter of recommendation to the Em- 
peror. It was especially magnanimous on the part 
of the Vuzeer to let bygones be bygones, since 
they included the murder, by himself, of his new 
Deputy's kinsman and former patron Moohummud 
Koolee Khan, the former Governor of Allahabad ; 
and it was not an impolitic stroke on the part of Sir 
R. Barker to lend his assistance towards introducing 
into the Imperial councils a chief who was as strongly 
opposed to the Rohillas as to the Mahrattas. 

Armed with these credentials, and accompanied 
by a small but compact and faithful force, the 
Meerza proceeded to court to assume his post. The 
newly- created premier noble, Zabita Khan, took 
refuge with the Jats ; but Hussam-ood-Dowla, who 
had been for some time in charge of the local 
revenue (Deeiean-i-Khalsa) Avas dismissed, put under 
arrest, and made to surrender some of his ill-gotten 



102 SKETCH OF THE HISTORY OF 

wealth. An inadequate idea may be formed of the 
want of supervision which characterized Shah Alum's 
reign, by observing that this man, who had not been 
more than two years in charge of the collections of 
a small and impoverished district, disgorged, in all, 
no less than fifteen lakhs of rupees.* He was suc- 
ceeded in his appointment by Abdool Ahid Khan 
(who bears henceforth the title of Mujud-ood-Dowla), 
while Munzoor Alee Khan, another nominee of the 
minister's, became Nazir, or Controller of the 
Household. Of these two officers, it is only neces- 
sary here to observe that the former was a Mussul- 
man native of Cashmeer, whose character was 
marked by the faithlessness and want of manly spirit 
for which the people of that country are proverbial 
in India ; and that the latter was either a very 
blundering politician or a very black-hearted traitor, f 

Mujud-ood-Dowla was the title now conferred 
upon the Cashmeerian, Abdool Ahid, whose pliant 
manners soon enabled him to secure a complete in- 
fluence over his indolent master. ISTujuf Khan seems 
to have been equally deceived at the time ; but after- 
events showed the difference between the undeceiving 
of a worn-out voluptuary, and that of a nature un- 
suspicious from its own nobility. 

Such were the first fruits of NujuPs alliance with 

* Probably as much as two years' land-tax on the same district 
now, although the value of money is, of course, very much fallen 
since those days. Perhaps it would not be an exaggerated estimate 
if the sum in the text were taken to represent a million and a half 
of our present money (sterling). 

t Vide inf , chap. v. p. 151, and chap, vi passiin. 



FORMATION OF A NEW PHASE 103 

obtained in the amorphous-solid condition, by very 
rapidly and strongly cooling its melt, and thus passing 
as quickly as possible through the region of tempera- 
ture, in which nuclei are easily formed. The mixture 
of silicates constituting ordinary glass has such a 
very slight tendency to crystallize, that we need not 
take any particular care to avoid the temperature 
region of rapid nuclear formation and rapid growth 
of the nuclei. But a glass may be devitrified by heating 
it long enough to high temperatures, at which crystal- 
lization proceeds more quickly. Silicates and borates 
are by no means peculiar in this respect ; many 
organic substances also form amorphous solids, when 
their melt is supercooled sufficiently rapidly. 

It is not quite easy to decide whether a substance is 
crystalline or whether it is amorphous-solid. External 
properties, such as vitreous nature and conchoidal 
fracture, are not very reliable. Precipitates formed 
from solutions are apt to show not the slightest indi- 
cation of crystalline structure, when examined under 
the microscope, and colloidal solutions may even 
evaporate to varnish-like pellicles, yet in either case 
the particles may have a minutely crystalline structure. 
Now amorphous solids differ from crystals in not 
showing a definite melting point ; they soften on 
heating and pass into the liquid state gradually, as 
is familiar in the case of glass. They therefore behave 
as if they were essentially of the same nature as 
liquids, only they are extremely viscous, so that their 
particles lack the mobility of a fluid. The molecules 



104 COLLOIDAL CHEMISTRY 

of amorphous solids are arranged, like those of a 
liquid, in haphazard fashion, and are not distributed 
with strict regularity in a space lattice, as are the 
molecules of a crystal. The above criterion of gradual 
softening is, however, not very definite, and the 
devitrification (crystallization) of many glasses on 
heating them is also troublesome. A remarkable 
difference between the crystalline and the amorphous- 
solid state is presented by their thermal conductivity, 
which in crystals rapidly increases at very low tempera- 
tures near the absolute zero, but in glasses diminishes 
in this region and becomes apparently constant near 
273. The utilization of this property is, however, 
not so simple experimentally, and is particularly 
difficult in the case of powders. 

X-rays on the other hand provide a quite general 
method for distinguishing crystalline from amorphous 
solids ; it is based on the following considerations. 
X-rays are light constituted by excessively short waves. 
The wave-length of visible light varies from 400 to 
800 pp, that of X-rays is only about o-i pp. When 
light passes through a fine uniform grating we observe 
so-called diffraction and interference phenomena : 
white light is resolved into a spectrum and mono- 
chromatic light, e.g. of a sodium flame, shows sharply 
defined light and dark lines, so-called interference 
bands. By looking at a source of light through a veil 
or through the eye-lashes, we can observe spectral 
colours, due to the same phenomenon, although 
not in a very pure form, Now von Laue made the far- 



FORMATION OF A NEW PHASE 105 

reaching discovery that the space lattice of a crystal 
can produce interference of X-rays just as a coarser 
grating, prepared mechanically, causes interference 
of visible rays. In order to obtain a readily 
observable interference, the distance between the 
rulings on the grating must bear a definite relation 
to the wave-length of the light. Now the distance 
between the atoms of a crystal is indeed such, that 
their arrangement in a space lattice may bring about 
the interference of X-rays. In contradistinction to 
the flat gratings, which are used for producing spectra 
of visible light, the crystal constitutes a spatial grating. 
The latter may be conceived to arise from a series of 
gratings or veils which have been arranged behind 
one another in regular fashion. The space lattice of 
crystalline substances causes a sharp interference of 
X-rays, which is wanting in amorphous solids and in 
liquids, since these have no strictly ordered space 
lattice. 

Debye and Scherrer have described a widely applic- 
able X-ray method, which is also especially suitable 
for the problems of colloidal chemistry. Fig. 18 
represents the apparatus diagrammatically. In the 
centre is a rod of the finely powdered solid, whose 
state of aggregation is to be determined. It is sur- 
rounded by a photographic film, contained in a lead 
chamber. The film serves to show the direction of 
the X-rays after their passage through the solid rod, 
for these rays cannot of course be seen, although they 
affect a photographic plate. The X-rays are as far 



io6 



COLLOIDAL CHEMISTRY 



as possible of uniform wave-length (i.e. " mono- 
chromatic," like the light from a sodium flame) and they 
enter through a window in the direction of the arrow. 
Interference occurs both in the passage and in the 
reflexion of the rays in the rod, and this interference 
causes pronounced lines on the film, as indicated in 
Fig. 18, but only if the particles constituting the rod 
are crystalline. If they are amorphous, we obtain 
instead of the sharp lines, a uniform darkening of the 
film or quite broad bands ; the same happens with a 




FIG. 1 8. 

liquid. Figures 19 and 20 of Plate I show the con- 
trast between amorphous freshly precipitated beryl- 
lium hydroxide and an older specimen which has 
become crystalline. Good varieties of glass are found 
to be completely amorphous ; their X-ray diagram 
shows uniform darkening of the film. 

The special application of Debye and Scherrer's 
method to colloidal chemistry will be discussed in 




FIG. 19. .PRECIPITATED BERYLLIUM 
HYDROXIDE 




FIG. -20. CRYSTALLINE BERYLLIUM 
HYDROXIDE 



FIG. 28. RAMIE FIBRES (S 



BROWNIAN MOVEMENT 107 

greater detail on pp. 133, 183, 184. In any case the 
X-ray apparatus described is an important requisite 
in a laboratory devoted to the study of colloids. 



C. THE BROWNIAN MOLECULAR 
MOVEMENT 

The importance of the Brownian movement in the 
study of colloids was already indicated in the intro- 
duction. It was discovered by the botanist Robert 
Brown, in 1827, when studying granules present in 
the contents of pollen-grains. As was also pointed 
out in the introduction, the Brownian movement 
must be regarded as due to the heat motion, and results 
from impacts of the molecules of the liquid with 
coarser visible particles. This interpretation of the 
phenomenon has only become certain during the 
first years of the present century, although the phe- 
nomenon itself had been known for a long time. For 
many years no special importance was attached to 
it, evidently because the movement was considered 
to be the accidental result of external vibrations or 
of heat currents. 

Many investigators were, however, impressed by 
the incompatibility of such a view with the fact that 
the Brownian movement takes place quite indepen- 
dently of external influences. It has been observed 
in a cellar which was so free from vibration that a 
mercury surface did not show the least disturbance, 
while the temperature was kept as constant as 



io8 COLLOIDAL CHEMISTRY 

possible ; not the slightest change in the movement 
was observable in the course of a year, in a micro- 
scopical preparation which was carefully protected 
against evaporation. Thus Chr. Wiemer was first 
led in 1863 to assume, that the phenomenon is caused 
by the blows of molecules of the liquid, which are 
postulated by the kinetic theory of heat. 

To this the following objection has indeed been 
raised. A single molecular blow would be too weak 
to cause any visible movement of a particle ; since the 
blows come quite at random from all sides, they 
should cancel one another and the suspended particle 
should remain at rest. As von Smoluchowski has 
pointed out, this is only true, however, when the 
interval of time during which the particle is observed 
may be considered as very long. In a game of chance 
a stake may be won or lost many times over during a 
given moderate interval of time. Not until the game 
has been played for a very long period do losses and 
gains cancel out completely. On applying the calculus 
of probabilities to the heat motion of the mole- 
cules of a liquid we find that the probable number of 
molecules, striking the particle in a given direction, 
is so large, that we may not regard our observations 
as extending over a very long period, but only over 
a certain moderately long time. During this time the 
blows in any particular direction may preponderate 
so greatly, that we can detect a movement of the 
particle. 

In 1906 Einstein and von Smoluchowski succeeded 



BROWNIAN MOVEMENT 109 

in basing a theory on this view, first put forward by 
Chr. Wiemer; the formulae deduced from the theory 
are in excellent agreement with experimental data. 
Einstein and von Smoluchowski started from the idea, 
that a coarser particle, exposed to the blows of the 
fluid molecules, would have to behave in just the 
same way as a large molecule which executes the heat 
movement spontaneously. Such particles therefore, 
when their motion makes them strike a wall, exert 
a pressure which must be equal to the osmotic pressure, 
that would be exerted by a solution of molecules of 
similar large size. The laws of van't Hoff, governing 
the osmotic pressure of dilute solutions, are actually 
found to hold also for suspensions and emulsions, in 
the manner which has just been suggested. Without 
the aid of mathematics it is difficult to explain how 
these laws were verified in the case of suspensions. 
We will only remark in general, that the Brownian 
movement is zig-zag and so irregular that the real 
velocity of a suspended particle cannot be determined. 
The movement may, however, be characterized by 
the so-called deviation of the particle. This is simply 
the length of the straight line joining the initial 
position to the final position reached in a given time. 
Fig. 21 represents diagrammatical] y the trembling 
haphazard movement of a particle, and every recti- 
linear portion of a zig-zag line is such a deviation. Now 
this deviation in its dependence on the viscosity of 
the fluid, on the radius of the particle, on the tempera- 
ture, etc., can be shown to behave quantitatively as 



no 



COLLOIDAL CHEMISTRY 



if the particle moved like a molecule of dissolved sub- 
stance, obeying van't Hoff's laws for dilute solutions. 
We may quote one more among the many deduc- 




FlG, 



tions of this theory which have been verified. The 
density of the atmosphere of course decreases as a 
result of gravity, the higher we ascend. Since a 
dilute solution behaves exactly like a gas, it is subject 



BROWNIAN MOVEMENT in 

to a similar gradation : the concentration of an other- 
wise uniform solution decreases in its upper layers. 
With delicate apparatus the decrease in the concen- 
tration of an electrolyte solution may even be demon- 
strated over a difference in level of one metre. The 
rate of decrease depends on the molecular weight of 
the solute ; it is most rapid (occurs at the smallest 
difference of level) when the molecular weight is 
greatest. In emulsions and suspensions, where, as 
was explained above, a very large molecular weight 
must be attributed to the " solute," a decrease in the 
concentration of suspended particles may already be 
observed at a vertical distance of a few thousandths 
of a millimetre. The numbers of particles, deter- 
mined microscopically at various heights in the liquid, 
are found to agree quantitatively with the theory of 
Einstein and von Smoluchowski. 



COLLOIDAL-DISPERSE SYSTEMS 

A. COLLOIDAL SOLUTIONS, SOLS AND GELS 
1. Sols 

GENERAL REMARKS 

AS was explained in the introduction, solutions 
may be regarded as colloidal, when they contain 
much larger particles than are present in ordi- 
nary true solutions. On the other hand their particles 
are still too small to be directly visible under the micro- 
scope, like those of a coarse emulsion or suspension. 
The diameters of colloidal particles are comprised 
between the limits of 1-500 //^. Of course there is no 
sharp line which separates colloidal solutions from true 
solutions on the one hand or from coarse suspensions 
and emulsions on the other. The transitions are quite 
gradual and continuous. Thus colloidal solutions are 
known whose particles are smaller than i ///i, and 
therefore of a size which is often attained by the solu- 
tion molecules of true solutions. Moreover many pro- 
perties of sols are also observed in coarse emulsions and 
suspensions, only in these the much larger particles 
usually settle down more rapidly, so that emulsions and 
8 113 



H4 COLLOIDAL CHEMISTRY 

suspensions are generally less stable than colloidal 
solutions. 

Although the line of demarcation is not sharp, we 
have generally no difficulty in deciding whether we 
are dealing with a colloidal, or with a true solution. 
Dialysis, already employed by Graham, is still one of 
the best means for this purpose. The solution to be 
tested is placed in a dialyser a small sack of parch- 
ment paper and surrounded by pure water, which is 
renewed from time to time. In the case of a coloured 
solute we can see directly whether or no it passes 
through the membrane. A colourless substance must 
be tested for by a suitable chemical reagent. 

The difference between the behaviour of colloidal and 
of true solutions may be rendered obvious by comparing 
the dialysis of a strongly coloured true solution, such 
as potassium permanganate, with a strongly coloured 
colloidal one, such as a gold sol. In the former case 
the outer liquid is soon coloured, in the latter case it 
remains colourless, even after a long time. 

There are many colloidal solutions, such as those of 
soaps and of many dyes, whose solute does indeed pass 
through the membrane, but much more slowly than 
in the case of true solutions. We are here concerned 
with relatively small colloidal particles, which approx- 
imate in size to the ordinary solution molecules. Such 
solutions are often called half-colloidal or semi-colloidal. 

Dialysis is simply a diffusion through a jelly, for as 
such the dialysing membrane must be regarded. In- 
stead of dialysing we can therefore directly observe the 



SOLS 115 

diffusion of the colloidal particles in a jelly. Test 
tubes are partially filled with a gelatine jelly, on 
which the solution under investigation is poured. In 
the case of a coloured solution we can see directly from 
the penetration of its particles into the jelly, that the 
solution is a true one. If it is a sol, the dissolved sub- 
stance does not diffuse at all, or only extremely slowly, 
but with true solutions the process is rapid. With 
colourless solutions we can employ the artifice of incor- 
porating in the jelly a substance which gives a char- 
acteristic chemical reaction with the diffusing substance. 

A further much used means of testing, whether or no 
a solution is colloidal, is provided by the ultramicro- 
scope. The limits of its application will be discussed 
later. 

A coarse emulsion or suspension is simply recognized 
as such under the ordinary microscope. 

A much more difficult problem than the delimitation 
of colloidal solutions as a whole is the classification of 
their many varieties. Fundamentally all classifica- 
tions are artificial, for every colloidal solution has its 
own peculiarities determined by the nature of the 
colloidal particles and of the dispersion medium ; this 
also applies to true solutions. Nevertheless many 
colloidal solutions have properties so similar, that they 
may well be considered and classified together. On 
the one hand there are those which, like aqueous 
solutions of starch and of proteins, are clearly more 
viscous than water and are little sensitive to electro- 
lytes ; even a fairly concentrated solution of sodium 



n6 COLLOIDAL CHEMISTRY 

chloride or sulphate hardly changes their appearance. 
This group of sols is often termed hydrophllic, because 
the colloidal solute is evidently related to water ; it 
readily imbibes water and retains it firmly ; the colloi- 
dal particles contain much water, which may be inferred 
from the great similarity of their refractive index to 
that of the dispersion medium. The viscosity of hydro- 
phobic sols, on the other hand, differs but little from 
that of water. These sols are very sensitive to electro- 
lytes ; of salts, like sodium chloride or sulphate, a 
decimolar solution already suffices to precipitate the 
colloidal particles as flakes, to coagulate them. This 
group includes the colloidal solutions of gold and of 
many other metals, of the sulphides and of various 
other substances. It will be seen that these are sub- 
stances to which we can hardly ascribe any special 
relationship to water ; they are indeed also hydro- 
phobic from the point of view developed on p. 57. 
Accordingly the colloidal particles of these solutions 
are not particularly rich in water and their refractive 
index is usually quite distinct from that of water. An 
artificial feature of this classification is, however, 
brought out by a consideration of the sols of ferric 
and aluminium hydroxides, vanadium pentoxide, etc., 
which are so sensitive to electrolytes, that in this 
respect they are absolutely hydrophobic, while on the 
other hand they are unmistakably related to water, for 
the refractive power of their particles differs but little 
from that of water, and the flakes which can be formed 
from these sols are generally strongly hydrated. Hence 



SOLS 117 

the designation hydrophobic does not seem to fit them 
closely. They are really intermediate between hydro- 
phobic and hydrophilic sols ; on account of their sen- 
sitiveness to electrolytes we will classify them among 
the former. 

When considering dispersion media, other than water, 
we may use the expressions lyophobie and lyophilic, 
instead of hydrophobic and hydrophilic. 

Another method of subdivision is into suspension 
colloids (or suspensoids) and emulsion colloids (or emul- 
soids). Here the criterion is intended to be the state 
of aggregation of the disperse phase : if the particles 
are solid, we have a suspensoid, if they are liquid, we 
have an emulsoid solution. Now in many cases the 
state of aggregation of the colloidal particles cannot be 
determined. This classification is therefore not based 
directly on a property which can be ascertained experi- 
mentally. Many authors are inclined to make the class 
of suspensoids co-extensive with that of hydrophobic 
sols, and that of emulsoids with hydrophilic ones. 
But this is certainly incorrect, for there are sols with 
liquid particles, such as an extremely fine emulsion of 
petroleum in water, which behave in a perfectly hydro- 
phobic manner. Conversely there is every reason for 
assuming, that the particles of many hydrophilic sols 
are amorphous-solid. Hence if we adhere, as some 
writers do, to the characteristics, which led to the con- 
ception of suspensoid and emulsoid, we may not iden- 
tify them with the concepts hydrophobic and hydro- 
philic. If, on the other hand, we follow other writers 



n8 COLLOIDAL CHEMISTRY 

in giving up the idea which originally led to the differ- 
entiation, there is no objection to using the terms sus- 
pensoid and emulsoid in the same sense as hydrophobic 
and hydrophilic. Since however no usage has yet 
become established which is consistent on this point, 
we will here only use the expressions hydrophilic and 
hydrophobic. 

A third method of subdivision is Into reversible 
(or resoluble) sols and irreversible (or irresoluble) ones. 
Thus an aqueous sol is resoluble, when after desiccation 
under certain experimental conditions the residue can 
be dissolved again to a sol by the addition of water ; 
it is irresoluble when this cannot be done. This 
method of characterizing sols seems at first sight very 
clear, but to the author's mind it suffers from the dis- 
advantage that it is not sufficiently general. Thus 
a concordant classification is not obtained unless the 
method of evaporation is stated very precisely. Again, 
a sol which at one time is found to be resoluble, may 
become irresoluble through the presence of a small 
quantity of foreign matter, which hardly alters the 
properties of the sol in any other way. The division 
between resoluble and irresoluble sols does not coincide 
with that between hydrophobic and hydrophilic ones. 
Many hydrophobic sols, such as that of gold, are indeed 
irresoluble, because their particles are not related to 
water ; but ferric hydroxide for instance, which on 
account of its sensitiveness to electrolytes, was reckoned 
above among the hydrophobic sols, is very often 
found to be resoluble. These three methods of classify- 



SOLS 119 

ing sols, into hydrophobia and hydrophilic, into suspen- 
soids and emulsoids, and into irresoluble and resoluble, 
are therefore not identical. In spite of various objec- 
tions, we will use in the following pages only the 
division into hydrophobia and hydrophilic sols. 

. \ PREPARATION AND PROPERTIES OF SOME SOLS 

The method of preparing hydrophilic sols generally 
presents few peculiarities. A solution of gelatin or of 
egg white can be prepared in the same way as any true 
solution ; other hydrophilic sols can easily be prepared 
from body fluids, such as an albumin solution from 
blood serum. Hydrophobia sols, on the other hand, 
are pre-eminently artificial laboratory products, and 
hence the preparation of a few of the most important 
will be briefly discussed. 

We have already alluded to the fact that the subject 
of colloids can be approached from two sides. We 
can either start from true solutions and enlarge their 
particles by forming a difficultly soluble precipitate, 
or we can take a liquid or solid in bulk and attempt to 
divide it up as finely as possible, to " disperse " it. 
The former is called a condensation method, and practic- 
ally all gold sols are made in this manner, The forma- 
lin method, which has been studied in special detail by 
Zsigmondy, may serve as an example. A feebly alka- 
line solution of a gold salt is prepared by adding to 
120 c.c. of very pure water (see p. 121) 2-5 c.c. of a hydro- 
chlorauric acid solution (made by dissolving 6 grm, 
HAuCl 4 '4H 2 in one litre of distilled water) and then 



120 COLLOIDAL CHEMISTRY 

3 c.c. of a 0*18 N solution of pure potassium carbonate. 
As soon as the solution has been heated to boiling, 
3-5 c.c. of a dilute formaldehyde solution are added 
(0-3 c.c. of commercial formalin in 100 c.c. of water). 
The following reactions then take place : 



HAuCl 4 4-2K 2 CO 3 + H a O 

2Au(OH) 3 + K 2 CO 3 

2KAuO a + 3CH a O + K 2 C0 3 = 2 Au + aHCOOK + H 2 O + KHCO 3 



The preparation of such a sol is largely determined 
by the rate of nuclear formation and the rate of crystal- 
lization, the most important properties of which have 
already been discussed on p. 94 et seq. From the gold 
atoms which are formed in the first instance, gold 
nuclei must result, which then grow out to gold 
crystals. The production of a colloidal solution is 
favoured by a high rate of nuclear formation, leading 
to the rapid production of so many nuclei that in the 
very dilute solution there is not enough gold available 
for them to grow to any considerable extent ; the 
particles therefore remain small and within the colloidal- 
disperse region. On the other hand a high rate of 
crystallization is unfavourable to the production of a 
colloidal solution, for it means that the first nuclei 
formed grow too rapidly to larger crystals, so that a 
coarse suspension results, instead of a sol. The rates of 
nuclear formation and of crystallization are greatly 
influenced by small concentrations of foreign sub- 
stances, in a manner as yet unknown. It is therefore 
necessary to follow a reliable recipe as closely as 



SOLS 121 

possible. Thus the purity of the water is important ; 
contamination with certain colloids (silicates, ferric 
oxide, etc.) is particularly harmful, probably because 
they lower the rate of nuclear formation or coagulate 
the particles already formed. It is necessary to use 
water which has been distilled through a silver or 
quartz condenser. The reducing agent must also be 
selected with a view to securing a high rate of nuclear 
formation ; this condition is sufficiently satisfied by 
formaldehyde. An ethereal solution of phosphorus is 
still better, but hydroxylamine is unsuitable, because 
with it the rate of nuclear formation becomes too small. 

The gold sols prepared in this manner are liquids of 
a beautiful ruby-red colour, which do not froth and 
differ but little from water in viscosity, surface tension 
and other properties. They are markedly hydro- 
phobic, i.e. very sensitive to electrolytes in small 
concentration ; when these are added, the red colour 
turns to blue and ultimately the gold is precipitated as 
a bluish-black powder. 

Another sol which has been much investigated, is 
that of arsenious sulphide. Analysts are familiar with 
the observation, that when hydrogen sulphide is passed 
into a pure aqueous solution of arsenious acid, the 
arsenious sulphide does not settle readily, but remains 
suspended in the liquid in a state of fine division. If 
a colloidal solution with particles of minimum size is 
desired, we must ensure, that the concentrations of the 
reacting substances are sufficiently small or that a suffi- 
cient number of nuclei are already present. We there- 



122 COLLOIDAL CHEMISTRY 

fore first add a dilute aqueous solution of hydrogen 
sulphide to a not too concentrated arsenious acid solu- 
tion and so produce a large number of nuclei. The 
reaction is then completed by passing hydrogen sul- 
phide gas. The- excess of this gas is removed by 
bubbling hydrogen through the liquid. This sol is 
characterized by the fact that in its preparation accord- 
ing to the equation 

As 2 O 3 +2H 2 S =As 2 S 3 +3H a O 

no electrolyte whatever is formed, which would other- 
wise have to be removed by dialysis ; hence there is 
no danger of coagulation. The As 2 S 3 sol is a clear 
pale-yellow liquid, which, if not very concentrated, 
resembles in colour a solution of potassium chromate. 

The sols of the metallic hydroxides (Fe(OH) 8 , A1(OH) 3 ) 
may often be prepared in a very simple and character- 
istic manner. Most salts of these hydroxides are 
so extensively hydrolysed in dilute solution, that on 
dialysis the acid diffuses away, while a sol remains 
inside the dialyser. A ferric hydroxide sol may there- 
fore be obtained by dialysing a dilute solution of ferric 
nitrate, because the latter salt undergoes hydrolytic 
dissociation according to the equation 

Fe(N0 3 ) 3 + 3 H 2 =Fe(OH) 3 + 3 HNO 3 

A commercial ferric hydroxide sol (ferrum oxydatum 
dialysatum) is prepared in pharmacy by cautiously add- 
ing ammonium carbonate solution to a solution of ferric 
chloride, so long as the precipitated ferric hydroxide 
still dissolves on shaking, and then dialysing the 



SOLS 123 

solution. The salt hydrolysis is therefore increased by 
suitably reducing the hydrogen ion concentration of 
the solution. 

So far we have been discussing condensation methods. 
These contrast to some extent with the so-called pep- 
tization or dispersion methods. By peptization we mean 
the " dissolving " of a coarse precipitate or of 
a continuous mass of a solid or liquid to form a 
colloidal solution. This often depends on the fact that 
the precipitate consists in reality of very fine particles, 
which by suitable treatment can be dispersed in a state 
of colloidal division. The peptization methods may be 
illustrated by the preparation of a vanadium pentoxide 
sol. Ammonium vanadate is ground up in a mortar 
with hydrochloric acid, so that vanadium pentoxide 
separates in red flakes according to the equation 

2NH 4 V0 3 +2HC1 -V 2 O 5 +2NH 4 C1 +H a O 

The pentoxide is washed on a filter until it begins to 
run through. It is then placed in a suitable quantity 
of pure water and shaken, until it has been peptized 
to a uniform sol of a fine reddish-brown colour. 

Metallic sols may further be prepared by the method 
of electrical disintegration, which was first used in its 
simplest form by Bredig:^ An electric arc is produced 
under water between stout metal wires, so that thick 
clouds of evaporated and disintegrated metal are 
formed, composed of particles so minute, that they 
mostly remain in colloidal solution. It is advisable to 
make the liquid faintly alkaline, for as we shall see 



124 COLLOIDAL CHEMISTRY 

later (p. 148), the hydroxyl ion favours the peptization 
of substances which tend to assume a negative charge, 
as most metals do. In this way aqueous sols of 
platinum, of gold and of silver are readily prepared. 
Svedberg has elaborated the method and made it more 
delicate. Instead of a direct current he employs the 
discharge from an induction coil, and chooses the 
variables of the discharging circuit resistance, self- 
induction, capacity and length of spark in such a 
manner, that sparks of minimum duration are formed, 
which transport a considerable quantity of heat. He 
could thus actually produce sols of the alkali metals 
in organic liquids. In this electric disintegration a 
variety of phenomena seem to occur, but the finest 
particles (which must be formed in order to get a stable 
sol) seem to arise chiefly by the evaporation of the 
strongly heated metal of the electrode. 

THE TYNDALL EFFECT AND THE ULTRAMICROSCOPE 

Many sols show a peculiar optical behaviour, even 
without special apparatus ; a casual observer might 
think that they fluoresce. This behaviour is seen very 
clearly when a powerful beam of light is sent through 
a sol in a dark room, and the beam is viewed side- 
ways. 

Whilst in true solutions, sufficiently free from dust, 
the path of the beam cannot be recognized, it is very 
obvious in colloidal solutions. The apparent fluores- 
cence depends on causes and follows laws, different 



SOLS 125 

from those of real fluorescence. 1 These phenomena 
are called after Tyndall, who first examined them 
more closely, the Tyndall effect. 

The sols which have been mentioned so far, have 
coloured particles and do not show the Tyndall effect 
in its simplest form. For this purpose it is better to 
use a colloidal solution or an emulsion with colourless 
particles, e.g. a mastic emulsion. The latter is pre- 
pared by pouring an alcoholic solution of mastic resin 
into an excess of water. If the somewhat milky 
liquid be examined for a Tyndall effect in the manner 
described above, the path of the light, when viewed 
sideways, is shown by a blue colour. All colloidal 
solutions with colourless particles show a blue Tyndall 
light ; indeed, this applies to all other colloidal-dis- 
perse systems, such as a fine smoke or milk glass, 
provided the particles are colourless. 

The phenomenon may be explained as follows. The 
light passing through the sol cannot, properly speaking, 
be reflected by the particles ; for this they are too small. 
In order to bring about a true reflexion the reflecting 
surface must at least have the dimensions of a wave- 
length, but the colloidal particles are smaller. They 
do, however, bring about, that a portion of the light is 
scattered laterally, and in this diffracted portion the 

1 It was formerly thought that the Tyndall effect could be 
very simply distinguished from true fluorescence by the fact 
that the light sent out laterally is polarized in the case of the 
former; and not in that of the latter phenomenon. Recently, 
however, the light sent out laterally in true fluorescence has 
also been found to be partially polarized. 



126 COLLOIDAL CHEMISTRY 

light of short wave-length, namely the blue and the 
violet, preponderates. Hence the beam of light, when 
viewed laterally, appears to be blue. If a sufficiently 
thin layer of such a mastic emulsion be viewed against 
a white background, it appears yellowish-red, for, as 
has been pointed out, the blue and violet portions of 
the light passing through are preferentially diffracted 
sideways, so that an excess of red and yellow rays are 
transmitted and make the liquid appear yellowish-red. 

The Tyndall effect is simply the so-called primordial 
phenomenon of Goethe, on which the latter based his 
theory of colour. He expressed it somewhat as follows : 
A turbidity seen against a dark background appears 
blue, against a light background it appears yellowish- 
red. The physicists of Goethe's time were unable to 
explain the phenomenon, which was first done by Lord 
Raylcigh in 1870, in the manner outlined above. No 
wonder that Goethe placed little confidence in the 
science of physics, because of its inability to explain so 
familiar a phenomenon, shown by any milk glass or 
smoke. 

The theory explains a further feature of the Tyndall 
effect, namely that the light diffracted exactly at right 
angles to the path of the beam, is completely polarized, 
i.e. this light vibrates in one plane only, perpendicular 
to the beam. If we look at the beam from any other 
direction, not at right angles, then the proportion of 
polarized light is smaller, diminishing according to a 
definite law along with the angle which the diffracted 
light makes with the beam. 



SOLS 127 

If a powerful beam of light, passing through a colloi- 
dal solution, is viewed perpendicularly through a suit- 
able microscope, we have the arrangement already 
referred to as the ultramicroseope. We have seen that 
every individual colloidal particle scatters light side- 
ways. In order to allow this light to be recognized, 
the particles must not be too closely crowded, for if they 
are, the light sent out by them overlaps, and we only 



T 

L, S I, 

FIG. 22. 

see a uniformly illuminated field. The colloidal solu- 
tion must therefore be suitably diluted, and the thick- 
ness of the layer under examination must be a small 
fraction of a millimetre. 

Fig. 22 represents the arrangement of the slit ultra- 
microscope, first constructed by Siedentopf and Zsig- 
mondy. A powerful beam of light from the source A, 
which may be the sun or an electric arc, falls through 
the lens LI on a rectilinear slit S, of which a much 



128 COLLOIDAL CHEMISTRY 

reduced image is projected into the inside of the cham- 
ber or trough T, by means of a second lens L a and 
an illuminating objective 0. The sol is contained in 
the chamber. The microscope M is so focussed, that 
the image of the slit comes into the middle of the field 
of vision. We therefore view an intensely illuminated 
and very shallow layer in the interior of the colloidal 
solution. A much used and convenient form of the 
chamber T was devised by Biltz and is represented in 




FIG. 23. 

Fig. 23 ; it allows the liquid to be run in and out with 
great ease. 

Zsigmondy has recently much improved the slit 
instrument by devising an immersion ultramicroscope, 
in which the illuminating and observing objectives are 
brought into the closest possible proximity. A 
chamber of the size used by Biltz is then unnecessary ; 
the liquid is in a small annular container between the 
two objectives. This arrangement minimizes the loss 
of light due to absorption in the colloidal solution. 

In the ultramicroscopes described so far the light 



SOLS 



129 



has to be carefully adjusted, in order to get good 
results. This is not necessary in other instruments, 
in which a special condenser, simply added to an 
ordinary microscope, produces dark-ground illumina- 
tion. Of these instruments we describe only the 
cardioid ultramieroscope. Fig. 24 is a diagrammatic 
representation of the condenser, which is introduced 
under the vertical tube of the microscope. Its central 
portion is impervious to 
light, which can only enter 
through an annular space 
(indicated by dotted lines) 
and is reflected in such a 
way, that after leaving the 
condenser the rays all inter- 
sect at one point below the 
microscope objective. The 
light emerges at such a small 
angle to the horizontal, that 
none enters the microscope 
directly ; it passes through a space (represented in 
Fig. 24 by the two upper plates) in which the col- 
loidal solution is placed. Usually a quartz chamber 
is employed, consisting of a slide and a coverslip, 1-2 p 
apart. The use of quartz has the advantage that the 
slide and coverslip can be carefully cleaned by ignition 
in a flame, for dust particles which are apt to reflect 
light into the microscope, are very troublesome, and 
only that light should enter the microscope, which is 
diffracted sideways from the colloidal particles. 





FIG, 24. 



130 COLLOIDAL CHEMISTRY 

SIZE, STATE OF AGGREGATION AND SHAPE OF 
COLLOIDAL PARTICLES 

' The size of the particles cannot be directly measured 
under the ultramicroscope, for of course we only see 
the light which they scatter, and the theory indicates 
that the apparent size of the particles depends on the 
aperture of the microscope. Nevertheless their actual 
size can be ascertained indirectly, at least under 
certain simple conditions. If the sol contains particles 
of one size only, we have merely to find the number in 
a definite volume of liquid and to determine the total 
amount of colloidal substance by suitable analytical 
means. In this way we obtain the total mass of the 
particles, and by division, the mass of a single particle. 
A knowledge of the density gives the volume of 
a particle, and if we know its shape, the required 
dimensions can be calculated. In the simplest case 
the particle is spherical and then its radius may be 
readily calculated. 

The lowest limit, at which colloidal particles can be 
recognized ultramicroscopically, is somewhat indefinite. 
It does not depend entirely on their size, but also on 
the difference between the refractive indices of the 
disperse phase and of the dispersion medium. If this 
difference is considerable, as in the case of an aqueous 
gold sol, then particles down to a radius of 5 pp may 
still be recognized with sufficiently powerful illumina- 
tion. Strongly hydrated particles, such as are present 
in many hydrophilic sols, can only be seen if they are 
much larger. 



SOLS 131 

Particles which can no longer be resolved under the 
ultramicroscope, are termed by Zsigmondy amicrons, 
those which are ultramicroscopically visible he calls 
submicrons and the still larger ones, revealed by an 
ordinary microscope, he terms microns. The last- 
named have a radius of more than 200 p/i. 

The following table, based on one by Zsigmondy, 
shows how the various properties change with the size 
of the particles and indicates where transitions occur. 

TABLE 2. 

o-i up i [if* 10 jw/i 100 pit i ft 10 fi ioo ft i mm 



Ultramicroscopic region ' Microscopic region 

Particles show Brownian No visible Brownian 

movement movement 



Particles pass through or- Particles retained by filter 

dinary niter paper paper 

Particles show increased Particles have ordinary 

solubility solubility 

solans 



An approximate idea of the size of colloidal particles 
may also be obtained by ultraflltration, a process which 
consists in forcing the dispersion medium through a 
suitable filter, by which the colloidal particles are 
retained. The frame work of the ultrafilter may be 
filter paper ; this is soaked in a solution of collodion in 
glacial acetic acid and then gelatinized by washing 



132 COLLOIDAL CHEMISTRY 

with water. The permeability of the ultrafilter may 
be diminished by increasing the concentration of the 
collodion solution ; it may be increased by soaking 
the filters in dilute alcohol. In this way various grades 
of permeability are obtainable ; the filters can be 
calibrated in respect of the diameter of their pores, 
by testing them on particles whose size has been ascer- 
tained with the ultramicroscope ; the filters can then 
be used to find the size of the particles of other sols. 
In using this method caution is necessary, because the 
fineness of the pores is not the only factor which holds 
back the particles : they are sometimes also held back 
by adsorption. 

Most colloidal solutions contain particles of widely 
different sizes, including amicrons in particular. They 
are multidisperse (polydisperse), and the determination of 
the size of their particles in the manner outlined above, 
is therefore not possible. If the particles are so coarse 
that they settle moderately quickly if we are therefore 
really dealing with suspensions the quantity of par- 
ticles of various sizes may be determined by measuring 
their rate of settling, which is connected by certain 
laws with their size. For this purpose the particles 
are allowed to settle on a scale-pan suspended in the 
sol ; the change in the weight of the pan is registered 
automatically. 

The preparation of colloidal solutions containing par- 
ticles of uniform size so-called unidisperse (monodisperse) 
sols is mostly very difficult, but for gold sols the 
problem has been largely solved by Zsigmondy. Sols 



SOLS 133 

with amicronic gold particles of a fairly uniform size 
may be obtained by reducing an auric chloride solution 
containing potassium carbonate by means of an ethereal 
solution of phosphorus. The particles may be enlarged 
uniformly by adding this amicronic sol, the so-called 
inoculation fluid, to another solution of a gold salt, 
which is undergoing reduction so slowly, that practic- 
ally no fresh nuclei are formed and that the liberated 
gold atoms are almost all deposited on the pre-existing 
nuclei of the inoculation fluid. A solution from which 
gold is liberated sufficiently slowly, may be prepared by 
adding a suitably small quantity of formaldehyde to an 
auric chloride solution containing potassium carbonate. 
The gold which separates is therefore deposited on the 
amicrons which have been added, and makes these grow 
quite uniformly, so that sols, containing exclusively 
particles of the desired order of magnitude, can be 
prepared. 

As long as the old opinion prevailed, that particles 
of colloidal solutions are always amorphous, there was 
a tendency to regard them as spherical in every case. 
The particles of many sols have however been shown 
to be crystalline (cf. p. 9), by means of the method 
of Debye and Scherrer (cf. p. 105). The particles of 
the sol are collected on an ultrafilter and moulded into 
a rodlet, which is examined by X-rays in the manner 
already described. The particles of many sols, such 
as those of gold, silver, ferric- and aluminium hydrox- 
ides, vanadium pentoxide, show more or less intense 
interference bands and are therefore crystalline. Other 



134 COLLOIDAL CHEMISTRY 

particles, of thorium hydroxide, sulphur and selenium 
sols produce a uniform darkening of the film, and hence 
are amorphous. 

In agreement with this we find that the particles of 
sols are by no means always spherical, but may have 
any shape. Thus the particles of aqueous vanadium 
pentoxide sols, which have been repeatedly referred 
to, are decidedly rod-shaped. This can be directly 
deduced from the ultramicroscopic image. These sols 
are moreover remarkable for a strong glittering under 
the ultramicroscope ; the particles flash into view and 
then disappear again. This is because the particles 
are only seen clearly, when their longitudinal axes are 
perpendicular to the direction of the incident light ; 
in any other position they are invisible. This rod-like 
shape of the V 2 O 5 -particles determines quite a num- 
ber of other phenomena. Thus the sols show a fluxional 
birefringence ('stream double refraction"). A crystal 
is of course called birefringent, when the light which 
passes through it in a certain direction, is split into two 
rays which are polarized in planes at right angles to 
each other, so that the vibrations of the two rays are 
at right angles. Such splitting occurs for instance, 
when a ray of light is suitably directed through a 
triangular prism, cut from a so-called uni-axial crystal 
in such a way, that the crystal axis coincides with the 
longitudinal axis of the prism. Now if we take a 
glass tube with triangular cross section, allow a V 2 O 6 - 
sol to flow through it and cause a ray of light to pass 
through it in the same way as in a crystal prism, then 



SOLS 135 

the light will be split into two rays which are polarized 
at right angles to each other, exactly as with a crystal. 
This is, however, only the case as long as the sol con- 
tinues to flow ; when it is at rest, the ray of light 
remains undivided, as happens in a liquid which is not 
birefringent. This is only one of the manifestations of 
fluxional birefringence, which may show itself in a 
flowing V 2 O 5 -sol in many other ways. It is due to the 
fact, that the elongated particles of the V 2 O 6 -sol, which 
are originally arranged in haphazard fashion, orientate 
themselves with their long axes in the direction of the 
stream lines. The orientation of the particles may be 
brought about by means other than a flow, namely by 
a magnetic field or an electric current. 

The two rays of light, passing through a crystal and 
polarized in different planes, may be absorbed to differ- 
ent extents. This leads to the phenomenon of dichro- 
ism ; the crystal presents a different colour, according 
to the plane of polarization of the light passing through 
it. Similarly V a O 6 -sols show a fluxional dichroism, 
which results in the peculiar streaks shown by these 
sols when they are stirred. Then we see, even with 
the naked eye, a multitude of yellowish, shimmering, 
silky streaks, like those sometimes seen in a liquid 
from which crystals are beginning to separate. When 
the sol has come to rest these streaks disappear again. 

A number of sols have properties similar to those of 
a V a 8 -sol. Thus under certain conditions the sols of 
benzopurpurin, cotton yellow and some other dyes may 
behave in the same way. As an example of sols with 



136 COLLOIDAL CHEMISTRY 

leaf-like, instead of rod-shaped particles, we may 
mention old ferric hydroxide sols. 



THE STABILITY OF HYDROPHOBIC SOLS 

Our knowledge of the conditions which determine the 
stability of colloidal solutions is still incomplete. As 
was pointed out above (p. 116) we can divide sols into 
two groups according to their stability : the hydropho- 
bic, which are unstable to small concentrations of 
electrolytes, and the hydrophilic, which are stable. Of 
course there is no sharp line of demarcation ; a number 
of sols occupy an intermediate position. Pronounced 
hydrophobic sols are those of gold, silver, platinum and 
other metals, provided they contain no protective 
colloids (cf . p. 157) ; further sulphur sols, prepared by 
pouring an alcoholic sulphur solution into an excess of 
water; the sols of arsenic and antimony trisulphides 
are also strongly hydrophobic. We will first discuss 
some of the more important peculiarities in the behav- 
iour of these hydrophobic sols towards salts. 

On adding an electrolyte solution of suitable concen- 
tration to such a sol under the ultramicroscope, we see 
that the Brownian movement is not primarily affected, 
but whereas before the addition of the electrolyte a 
particle, having approached another particle or the 
wall, did not adhere to it and continued its original 
motion unchanged, we now find, after adding the 
electrolyte, that the particle sticks to other particles 
or to the wall, The individual particles thus unite 



SOLS 137 

to larger and larger flakes, whose Brownian movement, 
on account of increased size, becomes weaker and 
weaker, and finally the flakes settle down on the 
bottom of the vessel. Th ; s process is called flocculation 
or coagulation. 

We therefore get the impression that the surface of 
the particles has been changed by the addition of the 
electrolyte. Originally the surface was not adhesive, 
but the electrolyte has made it so. Here the charge 
on the colloid particle is certainly an important factor 
and with many sols it is the only one. In discussing 
electrokinetic processes (p. 76) cataphoresis was des- 
cribed as the migration of the droplets of an emulsion, 
or of the particles of a suspension, to one of the poles, 
under the influence of an electrical potential difference, 
applied from without. Exactly the same is true of 
the particles of a colloidal solution, which migrate to 
the positive or negative pole and have therefore 
respectively a negative or positive charge ; hence we 
speak of negative or positive sols. Those of ferric and 
aluminium hydroxides, thorium dioxide, etc., are posi- 
tive, those of gold, silver, platinum, sulphur, the sul- 
phides, and vanadium pentoxide are negative. The 
discussion on p. 79 shows that the particles have no 
simple charge, but carry a so-called electrical double 
layer, and that the potential difference of the latter, 
the so-called electrokinetic potential, determines their 
behaviour. In cataphoresis and in other phenomena 
they behave, however, as if they had a free charge, 
which is accordingly referred to as such in this sense, 



138 COLLOIDAL CHEMISTRY 

Now there is the following connexion between the 
stability of the sol and the charge on its particles. 
As long as the charge is considerable and the particles 
migrate rapidly in cataphoresis, they do not adhere 
and the sol is stable. Probably the reason is, that 
when the particles approach each other closely, their 
double-layer is changed in such a way, that they repel 
each other. Now when an electrolyte is added to the 
sol, the electrokinetic potential difference is decreased 
in the manner already discussed, and the colloidal 
particles are therefore discharged. If the potential 
sinks to a certain small value complete discharge is 
unnecessary the particles show the peculiar behaviour 
described earlier in this section : they adhere on 
approaching each other sufficiently closely. This may 
be'explained on the assumption that their mutual repul- 
sion is much decreased, so that the mutual attraction 
of the particles asserts itself and they stick together. 
The nature of this attraction is still obscure ; perhaps 
it is of the same kind as that which reveals itself in 
crystallization and in adsorption. 

We can therefore readily understand, that the 
coagulation of hydrophobic sols by electrolytes is 
subject to the same influences as were described above 
(p. 84 et seq.) in considering the effect of electrolytes 
on the electrokinetic potential difference. Since in the 
latter case the discharge is principally brought about 
by the oppositely charged ion, we also find a similar 
preponderant influence of the oppositely charged ions 
in coagulation ; with positive sols the essential factor 



SOLS 



139 



is the nature of the anion, with negative sols that of 
the cation. The important features of this nature 
are the adsorbability and the valency of the ions, as 
the following tables show. The tabulated numbers 
are coagulation values, being the electrolyte concen- 
trations which produce a degree of change as com- 
parable as circumstances allow. Thus for the red 

TABLE 3. 

Coagulation values y of some negative hydrophobic 
sols, y in milligrammolecular weights per litre. 



Electrolyte. 


y for As 2 S,-Sol. 


y for Au-Sol . 


y for Congo- 
rubm Sol 


NaOH 






about 390 


NaCl . 


r 


24 


ios 


KC1 . .... 






90 


KNO 8 


5 


25 


IO2 


K 2 S0 4 

2 

HC1 


65-5 


23 
5c 


102 


Aniline hydrochloride . 
Morphine hydrochloride 
New fuchsine 
MgCl, 
CaCl a 


2-5 
0-42 
o-n 

0-72 

0-65 


o-54 

O-OO2 
O-4 1 


1-7 


SrCl 2 








BaCl a 


0*60 






U0 2 (N0 8 ) 2 .... 
A1C1 3 


0-64 

O-OQ3 


2-8 


O-245 


A1 2 (S0 4 ) 8 .... 

2 

Ce(N0 3 ) 8 .... 


0-096 
0-080 


0-009 
0-003 





140 



COLLOIDAL CHEMISTRY 



gold sol, which becomes blue on adding electrolytes, 
that concentration has been selected, which produces 
the blue colour within a specified time (5 minutes) ; 
in -the case of the arsenious sulphide sol it is that 
concentration, which causes the whole of the sulphide 
to settle down in flakes within two hours. 

TABLE 4. 

Coagulation values y of some positive hydrophobic 
sols, y in milligrammolecular weights per litre. 



Electrolyte. 


y for Fe..,O 3 -Sol 


y for Al/VSol. 


NaCl 


9'25 


77 


KC1 


9-0 


80 


BaCl 8 


9-65 










2 












KBr 


12-5 


150 


KI 


16 


about 300 


KNO 8 


12 




HC1 


> 400 





Ba(OH) a 


0-42 










2 






K-benzoate 





13 


K-salicylate 




8 


K-picrate 





4 


K 2 SO 4 


0-205 


0-28 


MgS0 4 


O-22 





K a Cr0 4 


_ 


0-60 


K a Cr a 7 


0-195 


. 


K a -oxalate ... 


_ 


0-36 


K e Fe(CN) s 





O-IO 


K 4 Fe(CN), 





0-08 



SOLS 141 

It will be seen that in the case of the arsenious 
sulphide sol there is very little difference between the 
values for KC1, KNO 8 and K a SO 4 , while those for 
KC1, BaCl a and A1C1 3 differ widely ; with the positive 
Fe a O 3 sol, on the other hand, there is very little 
difference between NaCl and BaQ 2 , while KC1 and 
K 2 SO 4 show a large difference. The diversity of 
the values for cations of the same valency, such as 
Na\ aniline-, morphine- and new fuchsine-ions, 
depends on the great differences in their adsorba- 
bility. 

The remarkable quantitative peculiarities of the 
coagulation values cannot well be explained without 
mathematical discussion. Here the following indi- 
cations must suffice : All experimental evidence so 
far suggests that the same discharging effect is obtained 
when equivalent quantities of the oppositely charged 
ions are adsorbed. The course of the adsorption 
isothermal (cf. Fig. 9) entails, that among ions of the 
same valency a much smaller concentration of a 
readily adsorbed ion is required than of one which 
is less adsorbable ; this brings about a large differ- 
ence in the coagulation values of ions of different 
adsorbability. In order to explain the influence of 
valency, it is not necessary to assume, that the ion of 
higher valency is adsorbed much more strongly ; if 
in equimolar concentration it were merely adsorbed as 
readily as an ion of low r er valency, this in itself 
would bring about, that much less of an ion of 
higher valency is necessary than of one of lower 



142 



COLLOIDAL CHEMISTRY 



valency ; this again results from the course of the 
adsorption isothermal. 

Perhaps Fig. 25 will make this clearer. Here the 
adsorption isothermal of cations of various valencies 
has been represented, for instance Na'-, Ba"- and 



I 



/-a 



FIG. 25. 



Al'"-ions. The ordinates a are the quantities of 
cation adsorbed by the colloidal particles, the 
abscissae (y-a) represent the concentrations of these 
cations in the solution. We must bear in mind that 
the coagulation value y does not take into account, 
that a certain quantity a of the cation has been adsorbed 



SOLS 143 

by the particles from solution and must therefore be 
subtracted from y, the total amount added. These 
concentrations, and the quantities adsorbed, are 
expressed in grammolecular and milligrammolecular 
weights respectively. We have therefore assumed 
that, reckoned thus in molar weights, the cations of 
different valency are adsorbed equally, and therefore 
the single adsorption isothermal of the figure is assumed 
to represent the behaviour of all three cations. Now 
coagulation is brought about by equivalent quantities 
of the adsorbed cations. Therefore, if the amount 
of univalent cations, which must be adsorbed for 
purposes of coagulation, be represented by the ordinate 
I, then the amount of bivalent ions to be adsorbed 
will be represented by an ordinate II of half this 
height, and that of trivalent ions by an ordinate III, 
one third as high as I. It will be seen that the (y-a) 
values corresponding to these three ordinates, show a 
much greater divergency than the ordinates them- 
selves ; the y- values also show correspondingly large 
differences, which are found to be quantitatively 
similar to those quoted in tables 3 and 4. 

In discussing the effect of electrolytes on the electro- 
kinetic potential difference (p. 84 et seq.) it was empha- 
sized, that under certain conditions ions may increase 
the charge (as well as diminish it). This phenomenon 
recurs in the coagulation of hydrophobic sols. Thus 
negative sols are not flocculated by small quantities 
of alkalies, but become more stable ; coagulation does 
not occur until the concentration of the alkali is very 



144 COLLOIDAL CHEMISTRY 

much increased (cf, table 3), when the influence of the 
cation outweighs that of the OH'-ion. Acids behave 
in a corresponding manner towards positive sols (cf. 
table 4). In very small concentrations many other 
ions charge up, instead of discharging, and this is the 
reason, why sols are generally not stable, when quite 
free from electrolytes. Indeed, a small quantity of 
an electrolyte, the so-called active electrolyte, is always 
necessary, in order that the particles may be provided 
with a sufficient charge. A colloidal particle may 
therefore be regarded as a complex, which owes its 
charge to an ion. The complex need by no means 
consist of a single constituent, such as pure silver in 
the case of a silver sol ; we should rather consider that 
a number of substances make up its constitution. Thus 
the particles of a silver sol, prepared by reduction with 
hydrogen, could always be shown to contain Ag 2 O. 
Moreover colloidal particles probably always contain 
water molecules ; in any case these would be held on 
the surface of the particles by adsorption. Zsigmondy 
represents the composition of colloidal particles by 
enclosing in brackets the constituents which belong 
to the complex proper. The ion which is in the dis- 
persion medium, and therefore forms the outer stratum 
of the electrical double layer of the particles, appears 
outside the brackets. The particles of a silver sol 
may therefore be represented thus : 



Here the anion, to which the charge on the particle is 



SOLS 145 

due, is not known with certainty ; otherwise it would 
be written at the end of the bracket. The H'-ion is 
pretty certainly the cation of the other stratum of the 
double layer. 

So far we have considered only the preliminary 
conditions which make the colloidal particles adhere, 
so as to form larger flakes. Thus we have taken into 
account some only of the processes, which occur during 
coagulation. The particles which have been changed 
by the addition of electrolytes unite to larger flakes 
at a certain rate, which depends on the degree to which 
they have been discharged. The simplest case is 
when the discharge has been complete. They then 
unite at a rate which can be calculated from their 
Brownian movement on the basis of the kinetic theory. 
Since in general two particles collide much more fre- 
quently than three or four, the coagulation takes place 
with a velocity corresponding to a bimolecular reaction, 
in which two molecules unite to form a chemical 
compound. 

The salient features of the coagulation of hydro- 
phobic sols, for instance the great influence of the valency 
of either the anion or the cation, extend to many other 
natural phenomena. Since this influence also occurs in 
the alteration of the electrokinetic potential difference 
by electrolytes, we cannot readily decide whether this 
alteration is the only cause, or whether a coagulation 
cooperates with it. The following example, in which 
flocculation doubtless also intervenes, may serve as 
illustration. The flagella of many bacteria are so 
10 



146 COLLOIDAL CHEMISTRY 

slender that they cannot be seen microscopically, 
unless they are placed in electrolyte solutions, when 
the slender flagella unite to form thicker tufts. In 
this phenomenon all the conditions recur which were 
described for the flocculation of negative sols ; the 
important factor is the valency of the cation. 

With those ions which are very strongly adsorbed, 
or have a high valency, and hence have a powerful 
discharging effect in small concentration, the following 
remarkable observations may often be made. Not 
only are the particles discharged, but their charge is 
reversed. Thus a negatively charged gold particle may 
be converted into a positively charged one by Th""- 
ions or by the cations of a basic dye-stuff. This more- 
over takes place so rapidly, that in passing through 
the zero value of the electrokinetic potential no floc- 
culation occurs, but instead a stable positive sol is 
obtained, containing the particles whose charge has 
been reversed. Not until a higher concentration of 
the electrolyte has been added, does coagulation occur, 
because the positive sol is then flocculated by the 
anion. If we therefore observe the behaviour of a 
sol on addition of increasing quantities of such an 
electrolyte, as in table 5 which represents the 
action of FeCl 3 on a negative Pt-sol, we obtain a 
so-called Irregular series : no flocculation at very low 
concentration ; then coagulation through the action 
of the cation on the negatively charged particles ; at 
still higher concentration reversal of charge, formation 
of a positive sol, no flocculation ; at the highest con- 



SOLS 



147 



centrations again coagulation, this time of positive 
particles by the anion of the salt. 



TABLE 5. 

Irregular series. 

Ferric chloride solution and platinum sol. 



Concentration of the FeCl, 
solution (milhmolar 
weights per litre) 


Occurrence of 
Flocculation. 


Direction of 
cataphoretic migration. 


0-0208 


No flocculation 


Migrates towards 


0-0417 


,, 


anode. 


0-0557 


Jt 




0-0833 


Complete flocculation 


Does not migrate. 


0-1633 


,, 




O-2222 


,, 




0-3333 


No flocculation 


Migrates towards 


0-5567 




cathode. 


0-8333 






1-633 






3-333 






6-667 






16-33 


Complete flocculation 




33-33 






83-33 






163-3 






333-3 






666-7 







The sol with reversed charge behaves exactly like 
a sol which is positive from the outset. 

As was briefly mentioned, the converse of coagula- 
tion is peptization, the process by which under suit- 
able conditions coagulated flakes may be divided up 
again into the original colloidal particles. (The term 



148 COLLOIDAL CHEMISTRY 

peptization is also employed in the more general sense 
of dispersing the coarse phase of a liquid or solid into 
particles of a sol. Here we will restrict its meaning 
to the disintegration of coagulated flakes.) Peptiza- 
tion may easily be characterized by definite numerical 
data. It is mostly carried out by washing the coagu- 
lated flakes with water, or with a suitable electrolyte 
solution. The peptization process is influenced by 
the age of the flakes, by the nature of the electrolyte 
originally used for coagulation, by the rate of stirring 
and by other factors, but the following general remarks 
may be made. It is certain that in the flakes the original 
particles do not at first fuse with loss of identity, but 
are separated by thin films of fluid. If the coagulating 
electrolyte is removed by washing, the particles may 
under certain conditions resume their original charge 
without outside interference ; they repel each other, 
and the flakes are broken up into the original fine 
particles. This succeeds most readily when the 
distinctive ion of the coagulant is feebly adsorbed or 
has a low valency, therefore especially with univalent 
inorganic ions such as Na', K', Cl' and NO 3 ' ; the 
more strongly the ion is adsorbed and the higher its 
valency, the more difficult is peptization. Those ions 
which are capable of increasing the charge on particles, 
have a pronounced peptic action ; hence for nega- 
tive particles this is so with the OH '-ion (as already 
mentioned on p. 143), further with the anion of citric 
acid; the H'-ion is especially active in peptizing positive 
particles. 



SOLS 149 

THE STABILITY OF HYDROPHILIC SOLS 

The hydrophilic sols with which we are primarily 
concerned, are typified by sols of agar and of starch ; 
gelatin and other protein sols will also have to be 
considered. The latter are markedly amphoteric, i.e. 
they are capable of forming salts both with acids and 
with bases, and this often brings about a special 
behaviour. In contradistinction to the hydrophobic 
sols, the hydrophilic are often very insensitive to 
electrolytes, such as the alkali salts of inorganic acids. 
Quite concentrated solutions of these electrolytes, 
several times normal, are required to produce any 
change, and then a separation of the colloidal sub- 
stance, a so-called salting out, is observed, such as 
also occurs with true solutions. This separation is 
determined by anions and by cations, both acting in 
accordance with their position in the so-called 
lyotropic series, discussed on p. 27. It was there 
emphasized, that the lyotropic series is doubtless 
closely connected with the degree of hydration of the 
ions. This suggests, that hydration is an important 
factor in the stability of hydrophilic sols, which sug- 
gestion indeed agrees with their behaviour in other 
respects. The stability of hydrophilic sols depends 
therefore on conditions, which are similar to those 
governing the solubility of a truly dissolved substance ; 
for with truly dissolved substances the interaction 
with the solvent also decides the solubility equilibrium. 
Although we have as yet no detailed information con- 



150 COLLOIDAL CHEMISTRY 

cerning these hydration equilibria, it is nevertheless 
probable, that a large water content of the colloidal 
particles generally enhances the stability of a sol. 

The stability does not, however, depend on this 
water content alone. Perhaps electrical influences, 
which were described in the previous section in 
explanation of the behaviour of hydrophobic sols, 
are also of importance to hydrophilic ones, together 
with the influence of hydration. In the case of agar 
sols it has actually been possible to separate the con- 
tribution, which the electrical charge makes towards 
the stability, from that due to hydration. Kruyt 
succeeded in discharging the particles by addition of 
an electrolyte and then obtained sols which retained a 
certain stability as the result of the water content of 
the particles, but were flocculated by dehydrating 
agents, such as alcohol. If, on the other hand, alcohol 
was added to an agar sol free from electrolytes, the 
stability due to hydration was abolished, and only the 
stability resulting from the electrical charge remained. 
Thus the sol became practically hydrophobic and was 
flocculated by electrolytes in accordance with the 
rules of coagulation. 

A further circumstance affecting the stability of 
hydrophilic sols is that in most cases their particles 
are appreciably smaller than those of hydrophobic 
ones. Whereas in the latter sols the diffusion of the 
particles can hardly be demonstrated directly, it is 
so considerable in many hydrophilic ones, that the 
rate of diffusion of their particles can be measured in 



SOLS 151 

the ordinary way. If for instance a layer of pure 
water is poured on a protein solution, the protein is 
found after some time to have diffused into the water. 
The molecular weight of the substance in colloidal 
solution may be deduced from its rate of diffusion ; for 
egg albumin a value of about 50,000 is found by this 
method, so that the radius of a spherical particle of 
this protein works out at about 3///J. 

In other cases it was found possible to ascertain the 
osmotic pressure of a colloidal solution directly. With 
true solutions this is of course measured with the aid 
of an osmotic cell, a vessel closed by a membrane 
which is impervious to the solute. It contains the 
solution and is immersed in another vessel filled with 
the pure solvent. In the case of most substances in 
true solution it is difficult to find good semipermeable 
membranes, although a membrane of copper ferro- 
cyanide happens to be suitable for sugar solutions. The 
osmotic pressure of colloidal solutions may be measured 
in the same way and here there are plenty of mem- 
branes possessing the required property of semi- 
permeability. All the membranes ordinarily used in 
dialysis (p. 114), such as parchment and pig's bladder, 
are available for this purpose. Nevertheless reliable 
measurements of the osmotic pressure have only 
been possible in few cases, more especially because 
the contamination of colloidal solutions by truly 
dissolved substances causes serious error, and may 
produce a considerable osmotic pressure, masking the 
smaller one of the colloidal particles themselves 



152 COLLOIDAL CHEMISTRY 

Among the more reliable measurements are those on 
haemoglobin ; here aqueous solutions gave a molecular 
weight of about 15,000, in agreement with the equiva- 
lent weight deduced from the iron content ; if the 
particles were spherical, they would have a radius of 
2fjLfji, which once more is a very small value. 

The small size of the colloidal particles of hydro- 
philic sols probably brings about that they are taken 
up by charcoal, kaolin, and other adsorbents in the 
same way as truly dissolved substances, in complete 
accordance with the rules of adsorption. Willstatter 
has recently utilized this fact for purifying ferments 
by fractional adsorption ; ferment solutions are essen- 
tially hydrophilic sols. The separation of substances 
by adsorption constitutes a very gentle treatment, 
particularly suitable for the purification of sub- 
stances which, like the ferments, are so very sensi- 
tive. 

In general therefore the hydrophilic colloids are 
more closely related to true solutions than are the 
hydrophobic. Among the hydrophilic, with soaps 
and many dyes, doubt may arise whether they 
are colloidal at all ; these, as we have already seen, 
are called semi-colloids. They actually dialyze through 
membranes, but very slowly ; they have molecular 
weights of not much over 4,000, at which value the 
colloidal properties begin to show themselves. In 
these semi-colloidal solutions we can distinguish, as in 
true solutions, between non-dissociated and dissociated 
particles ; the former are here colloidal, the latter 



SOLS 153 

are ionized into colloidal Ions and true ions. By col- 
loidal ions we mean electrically charged particles 
which are relatively small and therefore often present 
in considerable concentration ; they make an appreci- 
able contribution to the conductivity, lowering of 
vapour pressure, etc., and their concentration can 
therefore be determined by the methods in use for 
ordinary ions. In this way it has been possible to 
determine the proportion of colloidally and truly 
dissolved constituents of soap solutions, their content 
of true ions and of colloidal ions. Among many other 
solutions, which doubtless resemble soap solutions 
in this respect, those of salvarsan may be mentioned ; 
here the cation is colloidal, with soaps it is the anion. 
Markedly amphoteric colloids, like gelatin and the 
proteins generally, show many peculiarities. They 
form both colloidal cations and colloidal anions, the 
former chiefly in solutions containing excess of 
hydrogen ions, the latter in those containing hydroxyl 
ions in excess. If an electric current is passed through 
the solution of such a substance, the colloid migrates 
to the negative electrode in acid and to the positive 
in alkaline solution. In the neighbourhood of the 
neutral point the migration is less definite and the 
hydrogen ion concentration at which there is no 
migration at all, is called the Iso-electrlc point, and is 
characteristic of the particular colloid. At this point 
the sol contains the minimum quantity of colloidal ions 
and the colloid is composed practically entirely of 
neutral protein particles. The iso-electric point is 



154 COLLOIDAL CHEMISTRY 

distinguished by other features ; it represents the 
condition of minimum stability of the sol, in which 
it is most readily flocculated. This does not simply 
depend on the fact that hydrophilic sols, like hydro- 
phobic, are least stable, when they have been most 
fully discharged (cf. p. 138), for doubtless the influence 
of hydration is here even more important. All the 
experimental evidence points to the colloidal ions 
being much more strongly hydrated than the neutral 
protein-particles. Since hydration generally increases 
the stability, it is evident, that sols are least stable 
at their iso-electric point. At this point the viscosity 
is also a minimum, since the strongly hydrated colloidal 
ions materially increase the viscosity of the solution, 
and are here present in minimum amount. 

In connexion with the above we must consider a 
phenomenon, important in many respects, and termed 
Donnan's membrane equilibrium. It may be observed 
with special ease in solutions of colloidal ions, although 
the conditions determining it are present in almost 
all colloidal solutions. In a solution of Congo red, 
the anion is colloidal and does not diffuse through a 
collodion membrane, whilst the Na* cation passes 
through. Now if an osmotic cell contains Congo red 
and another salt, like sodium chloride, having the 
same cation, this salt does not distribute itself uni- 
formly between the liquids inside and outside the cell 
and quite independently of the Congo red, as one 
might expect offhand. On the contrary, the second 
law of thermodynamics leads to the conclusion, that 



SOLS 155 

when the concentration of sodium chloride is small, 
Congo red hinders the entrance of the salt into the 
cell ; more of it remains outside the cell than inside 
it. If the Congo red solution is dialyzed against pure 
water, we even see that the outside liquid becomes 
alkaline, whilst the dye solution becomes darker and 
turbid, as a result of the formation of the dye acid. 
This once more depends on the fact that the Na'-ion 
passes through the membrane, that of Congo red does 
not. In order that the Na'-ions may have an equiva- 
lent amount of anions, hydrolysis occurs ; OH'-ions 
also pass through the membrane, whilst an equivalent 
quantity of H'-ions are formed in the solution of the 
dye. Donnan's membrane equilibrium probably deter- 
mines the distribution of many ions between the 
various structures of the organism, 

THE MUTUAL INTERACTION OF DIFFERENT SOLS 

It is now fully recognized, that colloids are of decisive 
importance in many biological and technical pheno- 
mena ; yet the application of colloidal chemistry to 
these subjects often presents considerable difficulty. 
One of the chief reasons for this is, that both in nature 
and in the arts we are hardly ever concerned with 
structures containing a single variety of colloidal 
particle, but mostly with two or more varieties, and 
that the interaction between the colloidal particles of 
different kind presents many problems, as yet unsolved. 

The interaction between two hydrophobic sols is the 



156 , COLLOIDAL CHEMISTRY 

least complicated case. As a rule these do not coagu- 
late each other, unless their particles are oppositely 
charged. In order to obtain complete flocculation, 
the quantities of the two kinds of particles must be 
in a definite ratio, depending on the magnitudes of their 
respective charges. If this ratio is not maintained, 
if one or other kind of particle is present in excess, we 
do not observe any coagulation, or at most a slight 
turbidity, and the resulting sol has practically the 
properties of the variety present in excess. In this 
way we can for instance obtain a positive sol, by 
rapidly adding an excess of a positive sol to a negative 
one. This behaviour is therefore very similar to that 
found in the coagulation by strongly adsorbable ions, 
or by ions of high valency (cf. p. 146) ; there also a 
reversal of charge readily occurs, leading to an irregular 
series. The similarity is not merely formal, and depends 
on the fact, that the interaction of positive and nega- 
tive particles is subject to conditions, similar to those 
under which a particle takes up ions of opposite charge. 
Two hydrophobic sols with similarly charged 
particles generally do not affect each other. The 
properties of the mixture are such as might be expected 
from the proportion between the quantities of its two 
constituents. There are, however, remarkable excep- 
tions to this behaviour. Thus pronounced changes 
have been observed on mixing a negative sulphur or 
arsenious sulphide sol with a negative silver sol. There 
is a characteristic change of colour and sometimes 
flocculation actually occurs ; all this can only be 



SOLS 157 

explained on the assumption, that similarly charged 
particles have here also united to larger complexes. 
Probably it is the chemical affinity between the Ag 
atoms contained in the one kind of particle, and the 
S atoms in the other, which has made this possible, 
in spite of the charge on both particles being negative. 
The interaction of hydrophobic with hydrophilic 
sols is still more complicated. We might expect in 
the first place, that a hydrophilic sol is capable of 
imparting to a hydrophobic sol its own considerable 
stability towards electrolytes. This is actually the case. 
If, for instance, gelatin be added to a gold sol (such as 
is obtained by the reduction of an AuCl 3 solution by 
means of formaldehyde, cf. p. 120), concentrations as 
low as 0-005 or ' 01 m - m I0 c - c - of solution will be 
sufficient to prevent the change of colour from red to 
blue on the addition of 200 millimolar sodium chloride 
solution ; without gelatin a content of 20 millimols per 
litre is enough to bring about this colour change within 
5 minutes. Zsigmondy has termed this effect of 
hydrophilic on hydrophobic colloids a protective action 
and the quantity of the hydrophilic colloid, which 
under certain conditions protects 10 c.c. of the gold 
sol, he calls the gold number of the sol. Gold sols are 
of course not the only ones to be thus changed by hydro- 
philic ones. All hydrophobic sols which have been 
tested in this respect, are influenced in similar fashion. 
In most cases gelatin is specially active ; proteins are 
generally less effective and gum arabic, dextrin, etc., 
are still less so. 



I58 V COLLOIDAL CHEMISTRY 

The protective action is undoubtedly due, at least 
in part, to the adsorption of the hydrophilic colloid by 
the particles of the hydrophobic sol, so that the latter 
are entirely enveloped and acquire the properties of 
hydrophilic particles. An adsorption of gelatin by 
gold has been directly demonstrated. We do not 
know, however, what properties are responsible for 
the greater protective action of some hydrophilic 
colloids, as compared with others. It does not seem 
to be merely a question of adsorbability, but rather 
of the uniformity, closeness and solidity of the envelope, 
which the hydrophilic colloid forms round the hydro- 
phobic particles. 

This protective action has been applied in many 
ways. Thus the gold number serves to characterize 
and distinguish hydrophilic sols. We may recall the 
remarkable clinical observation that in certain diseases 
the cerebro-spinal fluid coagulates gold sols, whilst 
the normal fluid leaves them unaffected. The colloidal 
solutions of metals, e.g. of silver, which are used in 
therapeutics for various objects, are practically all 
protected by small quantities of hydrophilic colloids. 
Often fission products of proteins are used for this 
purpose ; thus the salts of lysalbic and of protalbic 
acids are particularly active. Unprotected silver sols 
would be liable to be precipitated by the electrolytes 
of serum and other body fluids. 

Some hydrophilic colloids, like saponin, protect 
hydrophobic sols at all concentrations, but many others 
only do so when their particles are present in sufficient 



SOLS 159 

excess, compared with the particles of the hydro- 
phobic colloid. If the hydrophilic colloid is present 
in smaller amount, we strangely enough do not observe 
any protective action, but the very opposite : , the 
hydrophilic colloid flocculates the hydrophobic one 
or at any rate it favours the flocculation of the latter 
by electrolytes, that is, in the presence of (these 
insufficient quantities of) the hydrophilic colloid a 
smaller concentration of a given electrolyte is required 
for coagulation than if the hydrophilic colloid is 
absent. The following will serve as an illustration. 
When a ferric hydroxide sol, and a solution of serum 
albumin, both as free as possible from electrolytes, are 
mixed, a clear, stable sol is obtained. The mixture 
is, however, far more sensitive to electrolytes than 
the pure ferric hydroxide sol after corresponding 
dilution with water ; its coagulation value for sodium 
chloride is for instance 1-2 milligrammolecular weights 
per litre, whilst that of the pure sol is 37 milligram- 
molecular weights. This phenomenon is termed 
sensitization. 1 Gelatin is remarkable in having in 
small concentrations a sensitizing action on many 
hydrophobic sols, such as neutral and acid sols of 
gold and of silver, and sols of many hydroxides ; in 
larger concentrations gelatin protects them. It is 
therefore incorrect to describe a colloid simply as 
protective ; according to its concentration and the 

1 This concept must be sharply distinguished from the 
specific, purely biological action, which serologists call sensi- 
tization. 



160 COLLOIDAL CHEMISTRY 

nature of the hydrophobia sol it may either protect or 
it may sensitize. 

No general explanation of sensitization has yet been 
given. In many cases the determining factor would 
appear to be the presence in the hydrophilic sols of 
colloidal ions (cf. p. 153), and their flocculating action 
on the particles of the hydrophobic sol, similar to the 
action of oppositely charged ions in general. It 
would therefore be a question of discharge by colloidal 
ions which bear a charge opposite to that of the col- 
loidal particles of the hydrophobic sol. In the above- 
mentioned example of the interaction between sols 
of ferric hydroxide and serum albumin the anions 
present in the latter solution would discharge the 
ferric hydroxide particles to such an extent, that the 
further discharge, required for flocculation, would be 
brought about by a smaller concentration of electrolytes 
than without the addition of the hydrophilic colloid. 
Cases are known, however, in which this explanation, 
seems to be inadequate. Thus tannin sensitizes both 
positive and negative sols of dye stuffs, and there is 
no reason for assuming, that a tannin solution con- 
tains both colloidal anions and colloidal cations. Here 
we are evidently not concerned with a discharging 
effect, but there is much that tells in favour of the 
state of hydration of the colloidal particles having been 
altered by the tannin, so that the stability of the dye- 
stuff sol would have been diminished in much the same 
way as was described above (p. 150) in discussing the 
stability of hydrophilic agar sols; the stability of 



SOLS 161 

these dye-stuff sols would therefore really depend in 
part on the state of hydration of their particles. 

Sensitization, like protective action, is susceptible of 
wide application. Since hydrophilic sols are apt to differ 
greatly in their capacity to sensitize, they may often 
be characterized in this manner. Thus the colloids 
of beer wort have been differentiated by this action, and 
by comparing them with colloids prepared from malt, it 
has been ascertained, that it is the gum of barley which 
has a decisive effect on the power of the wort to form 
a stable foam. It is further remarkable that a distinct 
difference is alleged to exist between the paraglobulins 
of normal and of pathological sera (e.g. in diphtheria and 
in tetanus) with respect to the power of sensitizing a 
ferric hydroxide sol. If this result were to be con- 
firmed, we should here have a case, in which sensitiz- 
ation reveals differences between sera, which can 
otherwise only be distinguished by biological means. 
Further the Wassermann reaction, and that of Sachs- 
Georgi, may amount to a sensitization, as was recently 
rendered probable by R. Stern. It seems very likely 
that luetic serum contains a substance, which favours 
the coagulation of the so-called extract-lipoids in a 
similar way to the above-mentioned action of tannin 
on the sols of dyes, without the colloidal particles 
necessarily being completely discharged. This sub- 
stance is always associated with the euglobulin ; it 
might also be, that in luetic serum a modification of 
the euglobulins has brought about a stronger sensitiz- 
ing action. In the Wassermann reaction the same 
11 



162 COLLOIDAL CHEMISTRY 

change in the luetic serum reveals itself by a greater 
adsorption of the complement, as a result of the forma- 
tion of complexes from the extract-lipoid and the 
luetic euglobulin. 

A further example of sensitization is provided by 
so-called agglutination. Many bacterial cultures, 
when suspended in water containing formaldehyde, 
yield a suspension which is markedly hydrophilic in 
character and is not flocculated by salts of the alkali 
or alkaline earth metals. Now typhoid (and other) 
bacilli may be changed into agglutinin-bacilli by 
inoculating an animal with the organism in question, 
and then treating the bacilli with the serum. This 
serum of the inoculated animal contains a substance 
termed agglutinin, which is taken up by the bacteria 
as if by adsorption, and renders the bacterial sus- 
pension hydrophobic. It is now precipitated by salts 
of the alkali metals, and of the alkaline earths, show- 
ing the same regularities of behaviour as have been 
described for the flocculation of negative sols. Table 
6 shows the coagulation values of a suspension of 
typhoid bacilli, of typhoid agglutinin-bacilli and, for 
purposes of comparison, of an arsenious sulphide sol. 

The agglutinin, which is probably present in colloidal 
form, has therefore had a sensitizing action on the 
bacilli. It should, however, be borne in mind that 
in one respect this agglutination differs greatly from 
the sensitizing actions discussed hitherto. It is 
extremely specific : typhoid bacilli are therefore only 
acted on by an agglutinin produced by inoculation 



SOLS 
TABLE 6. 



163 



Comparison of the coagulation of typhoid agglutinin 
bacilli with that of untreated typhoid bacilli, and of 
an arsenious-sulphide sol. 



Electrolyte. 


y of untreated 
typhoid bacilli. 


Y of typhoid ag- 
glutinin bacilli. 


v o|A A 


NaCl 


No coagula- 


2: 


CT 


NaNO 3 


tion 


25 




NaSCK 




c e 




2 

KOH 


No coagula- 


No coagula- 




HC1 


tion 

Q.C 


tion 
i 


OT 


HCOOH 
AgN0 3 - ... 
MgSO 4 


I'O 

25 
No coagula- 


i 
i 
i 'i 


O'8i 


CaCl 2 


tion 


2-7 


0*6^ 


Bad 2 


** 


2'5 


O'6Q 


Ni(N0 8 ) 2 .... 
HgCl, 


i '3 


i'3 
0*25 




A1 2 (S0 4 ) 3 .... 


0-08 


0-08 


0-096 


Fe 2 (S0 4 ) 3 .... 


0'2 


0-04 





2 









with typhoid, and not by inoculation with any 
other organism. It may be that a definite steric 
structure of the bacterial surface and of the agglutinin 
is required, in order that the agglutinin may fix itself 
on the bacterial surface so as to give to the latter a 
more hydrophobic character. In this connexion we 
may recall the special orientation of adsorbed mole- 
cules, which Langmuir has so successfully invoked 
in explanation of many phenomena (cf. p. 91). A 



164 COLLOIDAL CHEMISTRY 

steric structure of this kind might explain the great 
specificity of agglutination. 



PEPTIZATION BY HYDROPHILIC SOLS 

Many hydrophilic sols, especially semi-colloidal 
ones like soap solutions, have the power of emulsify- 
ing liquids, and of producing a suspension from solid 
substances. Although this power is connected with 
the protective action of the sols, it is undoubtedly 
distinct from it. As is well known, a pure oil, shaken 
up with water, yields a quite unstable emulsion, whilst 
a stable one is obtained, when the water contains a 
little soap, or when it is slightly alkaline, and the oil 
contains free fatty acid, so that a soap film can be 
formed at the interface between the oil and the water. 
The great activity of soap is shown by the following 
experiment : Oil is allowed to flow from a dropping- 
pipette, the nozzle of which dips below the surface 
of water, and the number of drops is counted which 
is furnished by a given volume of oil. Thus in one 
experiment 55 drops were obtained, both in pure 
water and in a slightly alkaline solution ; no larger 
number were obtained with acid olive oil flowing into 
pure water. But when the acid oil flowed into alkaline 
water the number of drops rose to 331, 

As with protective action, so also here the important 
factor is the adsorption of the hydrophilic colloid on 
the surface of the liquid to be emulsified, or on that 
of the solid to be suspended. But here, as in pro- 



SOLS 165 

tective action, the adsorption is not the only factor, 
for apparently those, colloids which are most strongly 
adsorbed, are by no means the most active, and 
undoubtedly other factors, as yet unknown, also play 
a part. A noteworthy feature is the power of forming 
solid or viscous envelopes at the interface, for an 
emulsion may also be stabilized by the addition of a 
suitable fine powder (such as soot and zinc hydroxide), 
if the solid powder forms a more or less continuous 
film at the interface. The hydrophilic colloid or the 
emulsifying powder is called the emulgent. 

When two liquids are emulsified together, the ques- 
tion keeps arising : which liquid is the disperse phase, 
which the dispersion medium ? When one liquid is 
in considerable excess it necessarily becomes the 
dispersion medium. But there is a fairly wide region 
of volume proportions, in which both water and the 
organic liquid (oil) may become the dispersion medium, 
according to the nature of the emulgent. The envelope 
formed by the latter presents its convex side to the 
dispersion medium, and the larger of the two inter- 
faces formed by the emulgent envelope will necessarily 
be on the side of that liquid, which most tends to 
have its interface increased by the emulgent. Hence 
the larger interface will for instance face the liquid in 
which the emulgent turgesces (swells), or by which 
the emulgent is readily wetted. For this reason 
hydrophilic emulgents, such as gelatin, gum, and 
alkali soaps, tend to make the water into the 
dispersion medium, whilst hydrophobic emulgents, 



i66 



COLLOIDAL CHEMISTRY 



such as resin soaps and the soaps of multivalent metals, 
favour an organic liquid as the dispersion medium 
(cf. Fig. 26). 

Further that liquid, which has the greater solvent 
power for the emulgent, also tends to become the dis- 
persion medium, for then the droplets of the disperse 
phase can build up their protective envelope from a 
relatively large store of the emulgent, whereas they 




ydropbilic 
'membrane 



!_i i i J I .' I J % 
Oraamc Liquid 

I l^f^-WrV^I l>| I 

Jtvdrophobic 
>mbrane 




FIG. 26. 

are less favourably situated if they have to rely on 
their own small store. The charge on the emulgent 
envelope also has some influence. A negative charge, 
which may be caused by OH'-ions, or by anions of 
hijgh valency, favours water as the dispersion medium ; 
a positive charge, resulting from cations of high 
valency or from cations of the heavy metals, favours 
the organic liquid. 
As will be seen from the nature of the emulgents 



SOLS 167 

referred to, there is a distinct possibility that when 
the amounts of the two liquids are in a suitable pro- 
portion, an oil- water emulsion may be converted into 
a water-oil emulsion. (The first-named word always 
indicates the disperse phase.) Thus an oil- water 
emulsion, prepared with alkali soap, may be changed 
to a water-oil emulsion by the addition of calcium- or 
zinc salts. These form calcium- or zinc soaps, which 
so strongly favour the oil as dispersion medium, that 
the one kind of emulsion finally turns into the other 
kind. The transformation is most readily detected 
by means of the electrical conductivity, which is 
greatly decreased by it. Such transformations are 
perhaps of biological importance. It is possible, as 
Clowes assumes, that the semipermeable protoplasmic 
membrane resembles such an emulsion, and that a 
change in the concentration of salts in the cell converts 
something resembling an oil-water emulsion into one 
of the opposite kind ; this might determine certain 
changes observed in the permeability of the proto- 
plasmic membrane, which are difficult to explain in 
any other way. 

The emulsifying action of hydrophilic sols is import- 
ant in other respects. The detergent action of soaps 
in washing depends essentially on the emulsification 
of greasy matter, which is thereby removed from the 
surface. The production of clay casts depends on the 
same principle. By stirring with alkali, clay and 
kaolin, which contain humus-like substances, are 
converted into a liquid sludge, which can be poured 



168 COLLOIDAL CHEMISTRY 

into moulds of plaster of Paris. The peptizing 
influence of the humic acid salts formed by the 
alkali results in the larger flakes of clay being disinte- 
grated into their finest constituent particles, thus 
producing a fluid sludge. 

2. Gels 

THE GENERAL PROPERTIES OF GELS 

At the present time the class of gels cannot be 
sharply delimited. The concept suggests in the first 
place jellies like those of gelatin and silicic acid, and 
so forth, and in the second place also structures like 
wool and cotton fibres, which are capable of taking 
up reversibly considerable quantities of water. The 
slightly hydrated flakes resulting from the coagulation 
of metal sols, sulphide sols and such like, will not be 
regarded as gels. For these flakes we propose to use, 
where necessary, the term coagulum, while we classify 
all jellies among the gels. It is, however, still uncertain 
whether all the structures classified above as gels, are 
really similar in character, and whether we shall not 
be forced at a later stage to draw some essential 
distinctions between them. 

Gels are often formed from sols, in many cases by 
coagulation. Thus an A1 2 3 sol sets to a gel on the 
addition of electrolytes, and the process follows closely 
the laws which govern the coagulation of hydrophobic 
sols (cf . p. 138 et seq.). The formation of a silicic acid gel 
from the corresponding sol also has the character of a 



GELS 169 

coagulation, but it resembles more closely the coagula- 
tion of a hydrophilic sol. These two processes are 
irreversible ; it is impossible to reproduce the sol 
simply by warming, or by washing away the electrolyte. 
On the other hand the formation of gelatin-, agar-, 
and soap gels is reversible : sufficiently concentrated 
colloidal solutions of these substances are sols above 
a certain temperature, and set to gels on cooling ; on 
warming these gels liquefy again and the transforma- 
tion can be repeated any number of times, if we neglect 
accessory factors and the chemical changes, which 
may occasionally affec these substances. This pro- 
cess is called a sol-gel-transformation (gelation), and 
should be sharply distinguished from a coagulation. 
In all these cases the gel is formed from a sol, and 
as was explained in the previous section, sols must 
in general be regarded as two-phase systems. Since 
during the conversion of a sol into a gel nothing 
occurs which might suggest the disappearance of the 
diphasic character, we are also obliged to regard gels 
as two-phase systems. Under suitable conditions 
ultramicroscopic examination leads to the same result ; 
if we observe the setting of a silicic acid-, gelatin- 
or agar sol, we see (provided the sol is not too concen- 
trated) a swarm of submicrons which at first are in 
lively Brownian movement. Individual particles 
undergo appreciable changes of position in the field 
of view, as is indeed also the case with sols. When 
gelation proceeds this translatory movement changes 
to an oscillatory one, in which the mean position of 



170 COLLOIDAL CHEMISTRY 

each particle remains fixed. This oscillatory move- 
ment becomes weaker and weaker, until finally it 
escapes detection. The flakes of the resulting gel, 
however, emit a bright Tyndall light under the ultra- 
microscope, and thus show that they consist of sub- 
microns and amicrons. The above applies to gels 
formed from dilute sols. Those formed from more 
concentrated ones are often quite incapable of ultra- 
microscopic resolution, for two reasons. In the first 
place the sols of silicic acid, gelatin and agar, which 
here come into question, often have strongly hy- 
drated particles, whose index of refraction does not 
differ sufficiently from that of the dispersion medium 
(cf. p. 130). In the second place the submicronic or 
amicronic particles of concentrated sols are generally 
only separated by amicronic liquid films having a 
thickness of 3-5 JLIJU, and such a structure is not resolv- 
able under the ultramicroscope. 

We have therefore some difficulty in directly demon- 
strating the diphasic nature of a gel, and hence some 
investigators prefer to regard the gels as one- 
phase systems. They assume that in a gelatin gel 
the individual particles are large molecules and that 
the water is, as it were, present in solid solution. Still 
other investigators lay stress on the fact, that a gel 
not only has a polyphasic character, recognizable 
under the ultramicroscope, but also a coarser hetero- 
geneity already visible under the ordinary microscope. 
A heterogeneity of this kind was observed by Biitschli, 
especially on allowing a silicic acid gel to dry, when 



GELS 171 

interstices, originally containing water or a solution, 
become filled with air. He therefore concluded that 
a gel has an alveolar or honeycomb structure, and that 
membranes, themselves consisting of submicrons and 
amicrons, surround cells filled with a liquid, or (in the 
desiccated gel) with air. According to Zsigmondy's 
experiments it is pretty certain that this microscopic- 
ally visible polyphasic character has nothing to do 
with the actual structure of the gel, but that it is due 
to gas bubbles. Although a framework of closed 
honeycomb cells is extremely unlikely in most gels, 
it is quite possible that there are often or always 
filaments or films composed of connected colloidal 
particles, and that these are of great significance in 
determining the elasticity and other mechanical 
properties of the gel. 

The statements on p. 130 concerning the size, shape 
and state of aggregation of the particles of a sol, also 
apply to the particles of a gel. Already quite a 
number of cases are known, in which the particles of a 
gel are rodlike, and hence probably consist of minute 
crystals. This is so for instance with the gels of soaps, 
of some urates and of many salts of the more complex 
alkaloids like quinine, eucupine, and ergotoxine. These 
alkaloidal salts separate from supersaturated solution 
as gels, and may on prolonged keeping be converted 
into larger crystals, which separate from the saturated 
solution. Such gels with crystalline particles show 
characteristic double refraction and dichroism, as 
was described for sols on p. 134 et seq. 



172 COLLOIDAL CHEMISTRY 

The interconversion of sol and gel is always a 
gradual, continuous process. On cooling there is 
never a definite ''freezing point," nor is there a definite 
" melting point " on warming, no matter what cri- 
terion be adopted. By rapidly cooling a sol it may 
often be kept fluid for some time at a temperature, at 
which the gel is the stable condition ; the sol then only 
sets gradually on prolonged keeping. It has been 
shown for soap gels and the same doubtless applies 
to many other gels that at one and the same tempera- 
ture there is in many respects no demonstrable dif- 
ference between the sol and the gel ; this applies to 
vapour pressure, electrical conductivity and other 
properties. It is only by its much greater solidity 
and elasticity, that the gel differs from the sol. 

We must not infer, however, that in contradis- 
tinction to gels, sols are quite inelastic. Those sols 
which are capable of setting, do indeed have a certain 
elasticity, although a much smaller one than the gels 
to which they set. This may be shown by observing 
the movement of a magnetic particle under the micro- 
scope. A minute fragment 1 of a magnetic metal, 
such as nickel, is placed in a sol under the microscope, 
and we observe how the particle is attracted by the 
pole of an electro-magnet. If the current be switched 
off, so that the attraction ceases, the particle does not 
remain at the spot to which it had moved, but recoils, 
sometimes even regaining its original position. This 

1 Unless the fragment is microscopic, it settles down too 
rapidly in the liquid, and is useless. 



GELS 173 

elasticity can be demonstrated in many sols (e.g. of 
sodium stearate and of gelatin) at concentrations so 
low, that the sol is only slightly more viscous than 
water. Highly viscous liquids, like glycerin or a 
concentrated sugar solution, do not show any trace of 
this elasticity. With gels the same experiments are 
possible, but of course the elasticity is here much 
greater ; a much larger force is required to give the 
same displacement to the nickel particle. Since so 
many sols are elastic, their viscosity cannot be 
measured with the ordinary apparatus ; the rate at 
which a sol flows from a capillary tube depends not 
only on its viscosity, but also on its elasticity. 

By means of these experiments with magnetic 
particles it is possible to measure not only the elasti- 
city, but also the elasticity limit, i.e. the limit to which 
the particle can be attracted without disruption of 
the sol or gel ; if this limit be surpassed the particle 
no longer springs back to its original position. This 
is the only indication that the elasticity limit has been 
exceeded ; the disruption itself is not directly observ- 
able. Experiments of this kind yielded the important 
result that the elasticity limit is especially high in 
" soft " gels, that is in those which are as it were 
intermediate between sols and gels ; the more dilute sols 
and the more concentrated gels both have a lower 
limit of elasticity. In the region of the high elasticity 
limit gels are apparently specially capable of being 
drawn out to filaments. 

In addition to the gelation of a sol as a whole, and 



174 COLLOIDAL CHEMISTRY 

to the separation of gel flakes, there is another way 
in which a gel can be formed from a sol. Very often 
thin pellicles of a gel are formed on the surface of 
colloidal or semi-colloidal solutions. The skin which 
is formed on the surface of boiled milk, is a well-known 
example. It is evident that this skin originates at 
the interface between liquid and air, for it can be 
lifted off without wetting the finger tips. Such 
pellicles are formed on the surface of many solutions of 
soaps, proteins, peptones and dyes. They may be con- 
sidered to originate as follows : The colloidal particles 
are first adsorbed on the surface, and becoming closely 
crowded, they are coagulated or undergo a sol-gel 
transformation, so that finally a pellicle of the gel is 
formed. Proteins apparently undergo chemical change 
in these skins, and are denaturated, so that they are 
no longer soluble in water. For this reason it is 
possible to remove protein from a solution by mere 
shaking, when care is taken to remove the foam each 
time it is formed, for every bubble of the foam is 
surrounded by a surface layer of protein, which has 
become insoluble. The production of foam implies 
the formation of an extensive surface, containing 
an abundance of protein, which is removed along 
with the foam. Such films are formed not only on 
the interface between liquids and gases, -but also on 
that between two liquids. Therefore they doubtless 
play an important part in the processes of cell 
division. 



GELS 175 

TURGESCENCE AND DETURGESCENCE l 

Gels may be divided into two groups, with reference 
to their power of imbibing a liquid and giving it up 
again ; those which can be made to swell, the turges- 
cible gels, and those which do not swell, the non- 
turgescible. An aqueous gelatin gel may serve as 
an example of the former class. On desiccation it 
loses water uniformly without the formation of 
vacuoles and finally the well-known horny gelatin 
plate is left. If the dry plate is immersed, water 
is taken up again uniformly, the total volume increases 
and finally we obtain a hydrated gelatin gel practic- 
ally identical with the original. The term turgescence 
will be confined to this kind of volume change, due 
to the imbibition of liquid, and deturgescence will be 
restricted to the converse phenomenon, the giving 
up of water. Other turgescible gels are those of agar 
with water, and of rubber with benzene, toluene or 
chloroform. 

An example of a non-turgescible gel is furnished by 
silicic acid. When the gel is first formed from the 
sol, it contains so much water, that its volume decreases 

1 A determined attempt should be made to fill a gap in 
English scientific terminology by inventing equivalents for 
the German quellen and its derivatives. This can be done by 
anglicizing the Latin turgesco, and using " turgesce " transi- 
tively for quellen. Turgescence and turgescible already exist 
in English and other derivatives can readily be formed. In 
deference to custom I have not entirely abandoned the rather 
unsatisfactory term " swelling," G. B. 



176 COLLOIDAL CHEMISTRY 

somewhat on drying. Beyond a certain definite point 
this no longer occurs, the external volume is pre- 
served and the further removal of water results in the 
interstices of the gel becoming filled with air or with 
water vapour. At first the gel is vitreous and semi- 
transparent, for its particles and the intervening pores 
are all amicronic or submicronic, and when this is so, 
we can indeed observe the Tyndall phenomenon, but 
no opacity. On further desiccation the gel assumes 
a white, chalky appearance. It is then permeated 
by microscopically visible gas bubbles, and the re- 
flexion of light from their surfaces makes the gel white 
and opaque, for the same reason that fine glass- 
powder is white and opaque. The gas bubbles are 
so large that each contains a considerable number of 
colloidal particles and of pores. On still further 
desiccation the gel once more becomes vitreous and 
semi-transparent ; the gas bubbles have disappeared, 
the pores are uniformly filled with air, and the small 
quantity of water still retained, envelops the colloidal 
particles as an adsorption layer. The optical con- 
ditions are therefore the same as at the beginning, 
except that the pores are now filled with air, instead 
of with water. Since they are amicronic, or anyhow 
of a fine submicronic character, the gel is again trans- 
parent. 

The intermediate range of hydration, in which the 
gel appears chalky, is termed the region of inversion. 
The process of desiccation may be studied quantita- 
tively, by measuring the vapour pressure of the liquid 



GELS 177 

contained in the gel. For this purpose the gel is 
placed in a desiccator containing dilute sulphuric acid 
of known vapour pressure, and the water content is 
determined, which the gel assumes in equilibrium 
with this pressure of aqueous vapour ; it may take 
months before a state of equilibrium is reached. The 
results of such experiments have led to the following 
conclusions : small quantities of water are held by 
adsorption on the colloidal particles ; the rest fills the 
pores of the gel, which pores are capillaries having a 
radius of 1-3 /^u. Since in such very narrow capillaries 
the surface of the liquid is strongly concave, the 
vapour pressure is much smaller than that on a plane 
surface at the same temperature. We may recall 
the increase and decrease of vapour pressure at convex 
and concave surfaces respectively (cf. p. 40). 

A silicic acid gel of this kind may be made to imbibe 
other liquids, organic or inorganic, and on this the 
technical application of similar gels is based. Dynamite 
is an example of this application ; nitroglycerine, 
imbibed by infusorial earth, is much less dangerous 
and more easily handled than in the liquid condition. 
Even if it undergoes decomposition locally, the heat 
given out does not raise the temperature sufficiently 
to cause an explosion, for the extensive surface of the 
colloidal particles absorbs much heat, and hence has 
a strong cooling effect. The great lowering of vapour 
pressure in such gels may occasionally be put to 
technical use. Thus a pungent and caustic liquid 
like bromine may be manipulated much more readily 

12 



I 7 8 



COLLOIDAL CHEMISTRY 



To Manometer 
and (738 Cylinder. 



for disinfection and other purposes, if it is imbibed 
by infusorial earth ; this is so-called " solid " bromine. 
In turgescible gels we must consider more particularly 
the turgidity pressure. The tendency of a dry, turges- 
cible gel to take up water is so great, that consider- 
able pressures, acting on the gel, may be overcome 
and conversely, a high pressure is required to squeeze 

water out of the gel. 
Fig. 27 represents an 
apparatus by means of 
which small turgidity 
pressures (of a few 
atmospheres) may be 
measured. Q is a disc 
of the gel, for instance 
dry gelatin or india- 
rubber. It lies in a 
glass tube G and rests 
FIG. 27. on the bottom of a 

porous cell T, which is 

immersed in the liquid having a turgescent action. 
The tube G and a portion of the calibrated 
capillary K are completely filled with mercury. 
K is joined to a cylinder of compressed gas, by means 
of which a known pressure can be exerted on the 
turgescent gel. The displacement of the mercury 
meniscus in the capillary indicates the amount of 
liquid taken up by the gel, and so the connexion 
between the turgidity pressure and the degree of 
turgescence (amount of swelling) may be studied. 




GELS 179 

In this way the turgidity of gelatin in water and of 
rubber in many organic fluids have been measured. 
The turgidity pressure increases very rapidly when 
the amount of fluid contained in the gel is small. 
Pressures of many thousands of atmospheres must 
be applied, in order to squeeze more fluid out of a 
gel, which is already fairly dry, and conversely enor- 
mous pressures may be overcome, when a very dry 
gel is allowed to turgesce. This was utilized by the 
ancients. Rocks can be split asunder by driving 
very dry wood into existing crevices, and then making 
the wood swell with a little water. 

In turgescence there is a considerable evolution 
of heat, the heat of turgescence, which is especially 
large for the initial small quantities of water imbibed 
by a gel. 

The nature of turgescence is still in dispute. Katz, 
who has studied the phenomenon most closely during 
recent years, considers it a process of solid solution ; 
thus in a gelatin gel water would be present in a state 
of molecular division between the molecules of gelatin. 
The similarity of the vapour pressure curves, when a 
turgescible gel and when concentrated sulphuric acid 
take up water, is indeed very striking, as Katz has 
rightly emphasized. On the other hand there is much 
justification in regarding a turgescible gel as a two- 
phase system in which the fluid has insinuated itself 
between the colloidal particles ; the imbibition would 
accordingly resemble an adsorption. There is indeed 
a distinct difference from ordinary adsorption, in so 



180 COLLOIDAL CHEMISTRY 

far as in the latter the interface of the adsorbent is 
not altered in extent, nor are the parts of the adsorbent 
interface appreciably displaced or rearranged. In 
turgescence, on the other hand, we must assume that 
the colloidal particles of the gel become disunited and 
that new interfaces are generated, or that the particles 
become separated by adsorption layers of increasing 
thickness, without the gel losing its cohesion. 

Turgescence is extremely important in many biolo- 
gical processes. There is every reason for assuming 
that muscles are to a large extent turgidity motors, 
and that the movements of plants are due to turges- 
cence and deturgescence. Biitschli and Spek have 
pointed out that the turgescence of two firmly united 
gel plates of different turgescibility may result in 
the formation of the pouches, which occur in the 
development of living beings, as for instance in the 
gastrula invagination. Indeed, the turgidity of living 
structures seems to have a definite influence in the 
formation of species. Thus Tower found in the case 
of certain beetles belonging to the genus Leptinotarsa 
which are related to the Colorado beetle, and inhabit 
dry districts of Mexico, that the crossing of related 
species resulted in quite different progeny, depending 
on whether the parent species did or did not live under 
the same conditions of moisture. Yet another biological 
application results from the following consideration. 
Just as the freezing point of a solution is lower than 
that of the pure solvent, so the liquid imbibed in a gel 
has a much lower freezing point than the free liquid. 



GELS 181 

At low degrees of turgidity, and therefore at turgidity 
pressures of a few thousand atmospheres, the freezing 
point may be lowered by 100 or more. This circum- 
stance is certainly important in determining the 
resistance to very low temperatures of organisms 
containing very little water, such as bacteria and 
spores. 

Now the process of turgescence in organized beings 
does not as a rule take place in pure water, but in 
aqueous solutions of electrolytes and of other sub- 
stances. Our knowledge of turgescence and of 
turgidity pressure in such solutions is by no means 
as extensive as we might wish. Present experience 
seems to indicate that in the turgescence of proteins 
the influence of H - and OH '-ions preponderates over 
that of all others and that both these ions greatly 
favour turgescence. 

These facts suggested to M. H. Fischer an explana- 
tion of the origin of oedema in kidney- and heart 
disease. He imagined that the connective tissue 
becomes acid and that its increased H'-ion concen- 
tration produces turgescence resulting in oedema. 
This theory is untenable. It was found that in the 
diseases in question the connective tissue does not 
become acid at all, and further, that in this particular 
tissue an increased H'-ion concentration does not lead 
to turgescence, but to deturgescence. Nevertheless 
turgescence remains important for the explanation 
of many processes taking place in connective tissue. 
Here we can only refer to the antagonism which exists 



i82 COLLOIDAL CHEMISTRY 

according to Schade between the ground substance 
of the connective tissue and the parenchyma cells 
imbedded in it. Influences which favour the turges- 
cence of the connective tissue, are adverse to the 
taking up of water by the parenchyma cells, and the 
converse also holds good. Thus for instance an 
increased H'-ion concentration has a deturgescent 
effect on the connective tissue, whilst the capacity 
of the cells for taking up water is increased ; an 
increased OH'-ion concentration has the opposite 
effect. In normal connective tissue there is there- 
fore a certain turgidity balance. 

THE ADSORPTION BY GELS. DYEING AND TANNING 

The colloidal particles of gels adsorb gases and 
dissolved substances according to the rules previously 
referred to (pp. 42, 58). Charcoal, which served as the 
principal example of an adsorbent, is indeed simply 
a non-turgescible gel ; for instance, it largely resembles 
a silicic acid gel (p. 176) in its power of taking up water. 
We may mention that in particular the adsorption of 
gases by a silicic acid gel has been investigated and was 
found to obey the well-known rules. The adsorption 
of substances of high molecular weight and of colloids 
from solution is influenced by the following circum- 
stance : The pores of the gel may be so fine, that the 
large molecules or the colloidal particles cannot pene- 
trate into the interior of the gel ; they remain on the 
surface and are there adsorbed. Thus the particles 



GELS 183 

of a silver sol merely cover the surface of a silicic acid 
gel and form a silver mirror ; something similar is 
observed in the adsorption by a silicic acid gel of 
colloidal solutions of ferric hydroxide, benzopurpurin 
and casein. 

The adsorption by gels is important in dyeing, 
tanning and related processes. Cotton-, silk-, and wool 
fibres are distinctly gel-like in character, In any case 
they are able to increase their water-content con- 
tinuously and reversibly, as in turgescence, and there- 
fore with a change in volume ; they also bind gases 
and dissolved substances as if by adsorption. Any 
attempt to explain dyeing processes is inadequate, 
which bases itself exclusively on a single aspect of 
the phenomena (whether wholly on adsorption or wholly 
on chemical combination), and neglects the great 
variation in the properties of fibres and of dye-stuff 
solutions. To illustrate the dyeing process we may 
consider more closely the dyeing of cotton by so- 
called substantive dyes like Congo-red and benzopur- 
purin. These colouring matters of course dye cotton 
without a mordant, while many other dyes, such as 
those of the triphenylmethane group, require that 
the cotton should first be mordanted in a suitable 
manner. Cotton, like most other cellulose fibres, 
has a decidedly crystalline structure ; examined by 
the method of Debye and Scherrer, it yields sharp 
lines (cf. Fig. 28 of the plate opposite p. 106). Cotton 
further resembles many crystalline adsorbents, in not 
adsorbing very strongly. Its chemical constitution 



184 COLLOIDAL CHEMISTRY 

makes an exchange adsorption (cf. p. 64) improb- 
able. The solutions of the substantive dyes are 
colloidal ; these substances must be reckoned among 
the semi-colloids, since their molecular weight is not 
large. They are adsorbed by cotton in accordance with 
the ordinary adsorption isothermal. Microscopic and 
ultramicroscopic investigation of the dyed fibre indeed 
shows, that the dye is chiefly present on the surface, 
and has not penetrated to any considerable depth. 
The important factor is, that the dye has been coagu- 
lated on the surface ; the dye bath of course contains 
salts in suitable concentration, which serve to secure 
this coagulation. Their concentration has to be 
selected in such a way, that coagulation only occurs 
on the surface of the fibre, where the amount of dye 
has been increased as the result of adsorption, and 
not in the bulk of the dye-stuff solution, for in the 
latter case dyeing would be adversely affected. Hence 
there is an optimum content of electrolyte, above and 
below which less good results are obtained. That it 
is really a question of coagulation results from the 
fact that in comparing different electrolytes, the 
nature and the valency of the cation are found to have 
a predominant effect, exactly as in the coagulation of 
negative sols (p. 139) ; the colloidal solutions of these 
substantive dyes are indeed negative. 

Of other fibres, silk, examined by the method 
of Debye and Scherrer, is found to be crystalline, 
and wool amorphous. Both adsorb distinctly more 
strongly than cotton, especially also substances in 



GELS 185 

true solution, not only azo-dyes and triphenylmethane 
dyes, some of which are entirely in true solution, but 
also alkaloids and their salts, mercuric chloride, etc. 
Silk and wool further have a marked tendency for 
exchange adsorption, which is in accordance with their 
chemical constitution ; as proteins they have a salt- 
like character, and it is quite possible for the ion of a 
dye-stuff salt to be exchanged for an ion of the fibre. 
Most probably the first stage of every dyeing process 
is an adsorption. This is followed later by another 
process, perhaps a purely chemical one, such as salt 
formation with the fibre or a chemical change of the 
dye in the adsorption layer, which second process 
makes the coloration into a true dyeing process. 
Examples are known in which these two successive 
stages can be clearly distinguished. Thus freshly 
precipitated flakes of aluminium hydroxide adsorb 
the blue acid of Congo-red, when they are shaken with 
the colloidal solution of this dye ; the blue flakes 
gradually become red, especially on warming, as the 
result of the formation of a red aluminium salt of the 
Congo acid. 

A similar succession of processes is found in tanning. 
The hide is an amorphous adsorbent, the solution of 
tannic acid (and of many other vegetable tanning 
agents) is semi-colloidal. The tannin is in the first 
instance taken up by hide powder according to an 
ordinary adsorption isothermal, but the adsorption 
compound so formed from hide and tannin, has not 
yet the properties of leather, It only acquires these 



186 COLLOIDAL CHEMISTRY 

gradually, becomes brown, no longer gives off tannin 
on washing, and becomes more resistant to dilute 
alkali. The adsorption is followed by another process, 
a chemical action or a coagulation, by means of which 
the adsorption compound acquires the properties of 
leather. 

DIFFUSION IN GELS. LIESEGANG STRATIFICATIONS 

Gels combine in a remarkable manner the properties 
of liquids with those of solid substances. The simi- 
larity to liquids shows itself inter alia by the fact that 
dissolved substances of not too large molecular weight 
diffuse in a dilute gel quite as rapidly as in a pure 
liquid. Common salt or urea diffuse in gels con- 
taining a few per cent, of gelatin or agar exactly as 
fast as in water. In concentrated gels, however, the 
diffusion of substances of high molecular weight is 
retarded, in comparison with their diffusion in pure 
liquids. The similarity to solids shows itself in the 
great elasticity of gels, mentioned above, and further 
particularly in the absence of convection currents, 
which are also absent from solids, but occur inside a 
liquid. The capillaries of a gel, in which the liquid 
is contained, are narrow, and a large part of the liquid 
is held as an adsorption layer, so that liquid currents 
are excluded. 

This combination of properties, which are elsewhere 
only encountered separately, results in the fact, that 
quite peculiar and characteristic processes may take 



GELS 187 

place in gels. On account of the relatively rapid 
diffusion, chemical interaction may take place in a gel 
as it does in a liquid. This chemical change might 
also be observed in solids, if it were not that the 
extremely slow diffusion in the latter generally prevents 
the detection of such interaction. Thus it is possible 
under suitable conditions to produce large and well- 
formed crystals in a gel, when in a liquid only a micro- 
crystalline powder is formed. The conditions are 
most favourable to the formation of large crystals, 
when a crystal nucleus can grow as freely as possible 
in a solution which is not too highly supersaturated. 
In a free liquid the corresponding conditions are not 
good ; any nucleus, which may be formed, falls to the 
bottom as the result of gravity and is thus removed 
from the uniform conditions obtaining in the interior 
of the liquid. There is further the possibility that 
several nuclei may unite, which likewise is unfavour- 
able to regular growth. In a gel, on the other hand, 
a nucleus once formed adheres by adsorption to a 
portion of the gel framework and grows out uniformly, 
because the substance present in supersaturated solu- 
tion only reaches the nucleus by diffusion. Hatschek 
was able to grow gold crystals up to 2 mm. in size, 
by allowing a suitable reducing agent, such as oxalic 
acid, to diffuse into a silicic acid gel containing auric 
chloride. Under other conditions of concentration 
gold leaflets, which are united together, may also be 
formed on the surface of the gel. All these structures 
(crystals, leaflets, etc.) closely resemble the metallic 



188 COLLOIDAL CHEMISTRY 

gold which has separated in nature. There is every 
reason to believe that the natural forms of gold have 
also been produced by the penetration of a reducing 
agent into an auriferous silica gel. 

A further example of this kind is supplied by the 
Liesegang stratifications, which occur most readily 
in gels. If two solutions, capable of forming a diffi- 
cultly soluble precipitate, are allowed to interact 
inside a gel, the precipitate is not always formed 
uniformly throughout the gel, but is often deposited 
in layers, These are repeated in regular fashion and 
are separated by intervals almost free from the preci- 
pitate. Liesegang stratifications have been examined 
most closely in the case of silver chromate. They 
may be prepared by allowing a sol containing 2-3 per 
cent, of gelatin and about o-i per cent, of potassium 
dichromate, to set on a glass plate ; a drop of concen- 
trated silver nitrate solution (50 per cent.) is then 
placed on the plate. The following reaction takes 
place : 

K 2 Cr a 7 + 4AgN0 3 + H a O = 2Ag i Cr0 4 + 2KN0 3 + 2HN0 3 
and the silver chromate separates in concentric bands, 
as represented in the frontispiece. 

An explanation first suggested by Ostwald and later 
modified somewhat, has proved correct. The principal 
factor is the rate of nuclear formation, the most 
important properties of which were discussed on p. 95 
et seq. It is of special significance to the present case, 
that this rate increases so rapidly with increasing super- 
saturation, At a low supersaturation the rate of 




LIESEGANG STRATIFICATIONS 



GELS 189 

nuclear formation is low, and few nuclei separate ; at 
a high supersaturation many nuclei are formed. When 
the silver nitrate solution penetrates into the gel 
containing the dichromate, the supersaturation in 
the foremost layer of the advancing silver nitrate 
solution is so small, that practically no nuclei are 
formed. A little further back the supersaturation 
is, however, already so large, that abundant nuclei 
result. These rapidly grow to crystals by the deposi- 
tion on them of silver chromate molecules, with regard 
to which the solution is supersaturated. The silver 
chromate molecules come not only from the region 
with abundant nuclei, but also from the zone in front, 
out of which silver chromate diffuses backwards to 
the nuclei. In this way there is formed in front of the 
zone of precipitation a region in which supersaturation 
is almost completely abolished, and in which no 
potassium dichromate remains. The concentrated 
silver nitrate solution now diffuses rapidly through 
this region, until in a region lying further forward and 
containing more potassium dichromate, an abundant 
formation of nuclei and precipitation once more occurs, 
whilst in the intervening space a few crystals at most are 
formed. Hence a ring of precipitate is always formed 
there, where the rate of nuclear formation reaches a 
sufficiently high value in consequence of a sufficiently 
high degree of supersaturation. 

The Liesegang stratifications are very dependent 
on the nature of the gel and of the precipitate formed 
in it ; this has been advanced as an objection to the 



igo COLLOIDAL CHEMISTRY 

theory just outlined, which does not predict this 
dependence. In many gels the rings are readily 
formed, in others not at all or imperfectly, and the 
nature of the precipitate also greatly affects the 
ease with which stratifications are formed. These 
differences, however, become intelligible, if we con- 
sider that the substance present in supersaturated 
solution is adsorbed by the colloidal particles, and 
that this adsorption greatly influences the nature and 
the rate of nuclear formation. Microscopic observa- 
tion seems to indicate, that the nuclei actually adhere 
to the colloidal particles of the gel and remain im- 
mobile, wherever they arise. 

If the above theory of the Liesegang stratifications 
is correct, the gel qua colloidal-disperse structure 
is not an essential requirement for the phenomenon. 
The only thing which matters is that a chemical reaction 
should take place in a liquid, undisturbed by currents. 
Thus it is actually possible to produce these stratifi- 
cations in the narrow spaces formed by pressing a 
cover-slip on a microscope slide. 

The importance of Liesegang stratifications in 
nature cannot be doubted. Thus the bands in agate 
are essentially of the same kind. Yet caution is 
necessary, when we desire to explain any banded 
vegetable or animal structure in this manner, for such 
banding may arise in a variety of other ways. Thus 
the patterns on butterfly wings must not be explained 
in this way, for they are already present in relief on 
the wings of the pupa, before the pigment has developed 



MISTS AND SMOKES 191 

The formation of folds and furrows, constituting the 
relief pattern, is therefore due to conditions of growth, 
and later the existing pattern is merely rendered more 
striking by the deposition of colouring matter. 

B. COLLOIDAL-DISPERSE STRUCTURES OF A 
DIFFERENT KIND 

1. Mists and Smokes 

Mists and smokes are colloidal-disperse structures 
in which the dispersion medium is gaseous, while the 
disperse phase consists of droplets (in the case of a 
mist) or of solid particles (in the case of a smoke). Just 
as coarse emulsions and suspensions are closely analo- 
gous to sols, so there is no need to separate off the 
smokes and mists containing microscopically visible 
particles from those consisting of particles of colloidal 
size ; the coarser structures are merely less stable, 
as a rule. Many other statements concerning sols 
are also applicable to smokes and mists. The latter 
arise either by condensation or by dispersion processes : 
either the disperse phase may separate from a super- 
saturated vapour, or a liquid or solid substance may 
be " atomized." The particles show a lively Brownian 
movement, more lively than in a sol, since the viscosity 
of a gaseous dispersion medium is of course less than 
that of a liquid one. The Tyndall effect is well marked 
and the ultramicroscope can be employed with success. 

One of the chief differences from sols is found in the 
electrical behaviour, at least in comparison with those 



192 COLLOIDAL CHEMISTRY 

sols, which have water as the dispersion medium. In 
water the number of ions is so large, that electric 
double layers, described on p. 138, are formed, and that 
we are unable to observe the taking up or giving off 
of single electrical charges by a particle. In a gas 
space the initial number of ions is small. If a mist 
or a smoke is generated in it, the particles are either 
uncharged or only charged to a slight extent, whether 
positively or negatively. The charge of individual 
particles may be altered by a definite, small amount, 
if we ensure that new carriers of electrical charges 
reach the particles and unite with them. In order 
to bring this about, we must increase the number of 
carriers of electricity in the gas space, the so-called 
gas ions, which can be done most conveniently by 
exposing the gas to radiation, for instance to X-rays. 
The gas is then ionized, that is, a large number of 
gas ions are formed. This phenomenon is very 
important, for by its aid it has been possible to measure 
the charge of an electron, i.e. the smallest quantity 
of electricity capable of existence. The number of 
such charges on a single mist- or smoke particle may 
be ascertained by observing ultramicroscopically how 
rapidly the particle moves upwards or downwards 
between two oppositely charged plates. 

The particles of coarse mists and smokes are de- 
posited under the influence of gravity. Here the 
other influences, such as that due to the electrical 
charge, become negligible, but in colloidal-disperse 
mists and smokes they play a real part. They have 



MID Id AINU DlYLUrUlD 1 93 

not yet been studied sufficiently closely, however, to 
allow of brief description. There is a further com- 
plication in the case of mists, since they may disappear, 
not only by the increase and consequent settling of 
their particles, but also by complete evaporation; 
thus an aqueous mist may disappear in a sufficiently 
dry space. If for any reason evaporation is prevented, 
the remarkable case occurs, of a mist persisting in dry 
air. This is the cause of the extraordinary stability 
of the notorious London fogs. Their droplets are 
coated with soot and with films of oily liquids, result- 
ing from imperfect combustion of coal ; this envelope 
prevents evaporation to such an extent, that the fog 
persists in a space which a hygrometer shows to be 
sensibly dry. 

The above example sufficiently illustrates the import- 
ant part which the stability of mists and smokes may 
play in meteorology ; the technical importance of this 
factor is also becoming more and more evident. In 
the Cottrell process mist and smoke particles are 
precipitated in large flakes at the surface of high 
tension electrodes (charged to about 100,000 volts). 
The process not only serves to remove smoke particles 
from furnace gases and so to combat the smoke nuis- 
ance, but also to collect technical reaction products, 
which, like sulphuric acid, are generated in the form 
of a mist. Another phenomenon of technical import- 
ance is the explosion of combustible dust, such as 
flour or coal dust. These dusts have been found to 
acquire considerable electrical charges, which may 

13 



I 9 4 COLLOIDAL CHEMISTRY 

lead to discharge by sparks ; in this way the finest 
colloidal particles are burnt, and the resulting flame 
may bring about the combustion of the whole cloud, 
provided the latter is thick enough, for the flame of 
each particle has to be communicated to neighbouring 
ones. The adsorption of oxygen by the finest dust 
particles seems also to be an essential feature of these 
explosions. 

2. Foams 

In the simplest case foams are disperse structures, 
consisting of a liquid dispersion medium and a gase- 
ous disperse phase. Whilst in most other colloidal 
structures the particles of the disperse phase are of 
colloidal minuteness, this is by no means essential, 
or even usual, in the case of foams : the gas bubbles 
of a foam are generally macroscopic or at least micro- 
scopic. On the other hand the dispersion medium 
is often e>f colloidal fineness, that is, the gas bubbles 
are separated from one another by liquid films, having 
a thickness of only a few /^a. Hence in a foam the 
surface of the liquid has been enormously extended, 
which is in opposition to the tendency of the surface 
tension to make the liquid surface a minimum (cf. 
p. 20). For this reason a liquid, which is to furnish 
a stable foam, must fulfil a number of special con- 
ditions. In the first place the surface tension of the 
liquid must be small, for otherwise its tendency to 
reduce the surface would be too powerful. Pure 



FOAMS 195 

liquids with a low surface tension, like ether and 
benzene, are, however, by no means specially inclined 
to foam. The reason is that at room temperature 
these liquids are all rather near to their boiling points ; 
they therefore have a high vapour pressure and films, 
composed of such liquids, evaporate too rapidly. A 
second condition for the production of stable foams 
is therefore that the vapour pressure should be small, 
as well as the surface tension. These conditions are 
fulfilled by aqueous solutions of capillary-active sub- 
stances (e.g. of amyl alcohol), and especially by 
solutions of many colloids and semi-colloids, like 
soaps, saponins, and proteins. In protein solutions 
a third influence plays a part, for they all have the 
property, referred to on p. 174, of forming solid pel- 
licles on their surface. These pellicles retard evapora- 
tion and surround the liquid lamellae with a frame- 
work, in the narrow lumen of which the liquid can only 
flow down very slowly. The quantity of liquid 
flowing through a capillary of course decreases 
extremely rapidly, when the bore is reduced. Since 
the rate of flow also depends on the viscosity of the 
liquid, it is evident that a certain degree of viscosity 
favours the stability of foams. 

Quite a number of conditions must therefore be 
satisfied, in order that a liquid may be capable of 
furnishing a strong and persistent foam. In the case 
of wort, the infusion of malt, before it is fermented to 
beer, two kinds of influences can be distinguished. In 
the first place there are substances, like albumoses, 



ig6 COLLOIDAL CHEMISTRY 

which cause strong frothing any how ; but the froth 
produced by them is not very persistent. The stability 
of the froth of beer is primarily due to the gum of 
barley, which increases the viscosity of the liquid. 

Definitely ultramicroscopic particles, such as are 
present in the above-mentioned gel-pellicles of soaps, 
saponins, etc., are not absolutely necessary for increas- 
ing the stability of the foam. This may also be done 
by much coarser microscopic or macroscopic solid 
particles, which remain on the surface, because they 
are only wetted with difficulty. The coarser particles 
also act by forming a framework, so that extremely 
fine channels are formed, through which the liquid 
only flows down very slowly. Such foams, rendered 
stable by coarser powders, play a part in the flotation 
process for separating ores, described on p. 58. 
The present discussion will show that the factors 
which stabilize foams are very similar to those which 
render emulsions stable (cf. p. 164^ seq.). In emul- 
sions stability is also promoted by soaps, saponins and 
other semi-colloids, forming solid pellicles, and by 
powders adhering to the interface. The cause is also 
the same in both cases, namely the support of the 
large interface of the foam or emulsion by a solid or 
viscous framework. 

The stable foams of soap solutions have been investi- 
gated in special detail, and also the properties of a 
single lamella of a soap solution, which properties may 
be observed by blowing soap bubbles with sufficient 
care. If such a lamella is made thinner and thinner, 



FOAMS 197 

the following changes are observed : At first the 
lamella is colourless, then brilliant colours appear, 
which are due to the interference between* the light re- 
flected from the two surfaces of the lamella. Reference 
has already been made (pp. 37 and 105) to the forma- 
tion of such interference colours by diffraction from a 
grating, or by thin oil films. On still further reducing 
the thickness of the soap lamella, remarkable black 
patches are formed, which represent the thinnest 
films of soap solution, capable of existence. The 
thickness of these films is of the order of 5 to 15 /*//. 
The black patches border on a much thicker lamella, 
so that at the boundary there is quite a sudden change 
of thickness. Within the black patches black lamellae 
of varying thickness may be distinguished, which also 
pass into each other discontinuously. Probably these 
black patches consist exclusively of the viscous surface 
layer, which on other grounds must be assumed to 
exist on the surface of soap solutions (cf. p. 174), 
and since these layers consist of only a few strata of 
molecules, there is the possibility of the existence of 
black patches of varying thickness, each composed of 
a different number of molecular strata. The relatively 
high stability of these extremely thin films probably 
depends on the fact that soap sols and soap gels 
contain pre-eminently filamentous particles (cf. p. 
171). These filaments are only a few molecules in 
thickness, but very long, and form a strong felt work 
in the gel films, giving support to the extremely thin 
lamellae. 



ig8 COLLOIDAL CHEMISTRY 

3. Disperse Structures having a Solid Dispersion 
Medium or having more than Two Phases 

Among colloidal-disperse structures with a solid 
dispersion medium certain glasses deserve further 
consideration. Gold ruby-glass is not only one . of 
the oldest technical colloidal products, first prepared 
by Kunkel more than 200 years ago, but also figures 
prominently in the history of colloidal chemistry. It 
was one of the first cases in which Siedentopf and 
Zsigmondy proved the existence of colloidal particles 
by means of their ultramicroscope. Gold ruby-glass 
is prepared by dissolving a little gold salt in melted 
lead- fcr baryta- glass ; on rapid cooling a colourless 
glass is obtained, which probably does not contain 
gold in chemical combination, but in supersaturated 
true solution. On heating once more to a high tempera- 
ture, the supersaturation is abolished and the gold 
separates in the glass in a state of colloidal dispersion. 
Under the ultramicroscope the appearance is very 
similar to that of a gold sol, except that the particles 
remain motionless in the glass, and show no Brownian 
movement. If the glass be heated too strongly or too 
long, its colour changes to blue and brown, the gold 
particles are coagulated, and become coarser, if the 
glass is sufficiently fluid ; thus the same colour changes 
occur which have been described for the coagulation 
of a gold sol (p. 12 1) . Besides the gold ruby-glass, other 
ruby glasses may be prepared in similar fashion with 
silver, copper, selenium, etc. With copper distinct 



SOLID DISPERSION MEDIA 199 

varieties with particles of various sizes are manu- 
factured : the red ruby-glass proper, with ultjramicro- 
scopic particles, the opaque copper-coloured haema- 
tinon with microscopically visible particles, and 
Aventurine glass, which contains copper spangles 
visible to the naked eye. 

A further group of similar structures is represented 
by the so-called coloured salts, which owe their colour 
to metals present in a state of fine division. Thus if 
a carefully dried crystal of sodium chloride be heated 
in sodium vapour at a temperature somewhat below 
the boiling point of the metal, it is coloured yellow or 
sometimes brown and green. Similar colours are 
produced when a crystal is exposed to X-rays, cathode 
rays or the radiation from radio-active substances, 
which decompose the salt with separation of the metal. 
Under the ultramicroscope these coloured salts show 
particles, quite similar to those in ruby glasses, and 
hence there is every reason to conclude that they 
contain colloidal-disperse metal. Coloured rock salt 
also occurs in nature ; it is generally blue, but other 
colours, such as violet, green and yellow may occur. 
The behaviour of these natural coloured salts is so 
similar to that of the artificial ones, both under the 
ultramicroscope and in other respects, that the 
natural colours are probably also due to colloidal- 
disperse metal, set free by radio-active substances in 
the neighbourhood. 

This group of structures also comprises the important 
photohalides. This name is applied to silver halides, 



200 COLLOIDAL CHEMISTRY 

containing silver in a state of colloidal division, in 
allusion to the fact, that they are especially formed 
from silver halides by exposure to light ; this exposure 
brings about a partial reduction to silver, which first 
separates in a colloidal-disperse condition with the 
unchanged halide as dispersion medium. The phato- 
halides are variously coloured yellow, purple or 
violet, according to their content of free silver and 
the shape of the silver particles. The substance 
constituting the latent image of an undeveloped photo- 
graphic plate is without much doubt simply a photo- 
halide of such low silver content, that it cannot be 
recognized by its colour ; the silver particles are never- 
theless able to act as nuclei for the deposition of further 
quantities of silver, formed by the reducing action 
of the developer. 

We will mention only two of the remarkable pro- 
perties, which distinguish photohalides, and particu- 
larly photochlorides, that is, colloidal solutions of 
silver in silver chloride. In the first place they have 
the property of colour adaptation. If a photochloride 
is exposed to coloured light, it acquires to a large 
extent the colour of the particular light, with which it 
has been illuminated ; a kind of colour photography 
is achieved in this way. The blue and red colours 
especially are reproduced with some approach to 
accuracy. A still more surprising phenomenon is 
the photodichroism of photochlorides, discovered by 
Weigert. If these substances are exposed to plane 
polarized light, a red spot is produced. This, viewed 



MORE THAN TWO PHASES 201 

through a Nicol prism, is found to be dichroitic ; it is 
brightest when the plane of polarization of the light, 
originally used for illumination, coincides with that 
of the light by which the spot is viewed through the 
Nicol prism. It would take us too far to explain, 
how. according to Weigert photodichroism and other 
related phenomena are connected with the colour 
adaptation of photochlorides, and how this group of 
phenomena is the basis, not only of the numerous 
examples of colour adaptation in nature, but also of 
the action of light on the retina of the eye. 

It will be evident from what has just been said 
about photohalides, that the chemistry of photo- 
graphy is very largely based on applications of colloidal 
chemistry. Indeed, the photographic plate turns out 
to be a specially complicated colloidal structure, in 
which no less than four phases must be distinguished ! 
In addition to the gelatin, and the aqueous solution 
in which it swelled, there is the photobromide, itself 
consisting of silver bromide and free silver. This 
large number of phases, capable of colloidal-chemical 
(and chemical) interaction in various ways, makes it 
so difficult to explain the processes occurring in a 
photographic plate. A large number of structures, 
important alike in nature and in the arts, present the 
same difficulty. Thus wood contains, apart from the 
turgidity water, both cellulose and lignin ; starch 
contains amylose and amylopectin ; in bread we must 
distinguish colloidal particles, as well as the coarser 
ones of crust and crumb, in which the starch grains 



202 COLLOIDAL CHEMISTRY 

are moreover embedded in a matrix of coagulated 
protein. A similar polyphasic character must be 
assigned to almost all natural membranes and fibres. 
At present the elucidation of the varied behaviour of 
such complex structures presents great difficulties, 
rarely overcome. Not until this has been achieved, 
however, will it be possible to treat many technical 
and biological problems successfully from the stand- 
point of colloidal chemistry. The investigation of 
these polyphasic structures is essentially the task of 
the colloidal chemistry of the future. 



INDEX 



AUTHORS AND SUBJECTS 



Active electrolyte, 144 
Additive behaviour, 27 
Adsorbability by different 

adsorbents, 63 
Adsorbate, 45 
Adsorbent, 45 
Adsorption, 31, 35, 58-71 
apolar, 64, 66 
exchange, 64, 65 
of gases, 42 
by gels, 182 
polar 64, 66 
in solution, 58-75 
Adsorption catalysis of gas 

reactions, 50 
in solution, 71 
Adsorption equilibrium, 44, 

59 
Adsorption isothermal, 44, 

60 
Adsorption theorem of Gibbs, 

33, 49, 61, 63 
Adsorptive, 45 
Agar, hydrophilic sols of, 

150 

Agate, 190 
Agglutination of bacteria, 

162, 163 



Alkali metals, sols of, 124 

Aluminium hydroxide sols, 
122, 137, 140 

Amicrons, 131 

Amino-acids, catalytic oxida- 
tion of, 73, 74 

Amoebae, ingestion of solids 

by, 58 
Amorphous-solid state, 102- 

106 

Angle of contact, 23, 58 
Antimony hydride, see sti- 

bine, 50 
Antiseptics, adsorption of, 

70 
Aplysia, adsorption of vera- 

trine by heart of, 70 
Apolar adsorption, 64, 66 
Arsenious sulphide sols, 121, 

139 
Assimilation of carbon by 

plants, 74 
Atomic structure, 5 



Bacteria, agglutination of, 

162, 163 
flagella of, 145 
203 



204 



COLLOIDAL CHEMISTRY 



BlLTZ, 128 

Birefringence, fluxional, 134 
Blood corpuscles, adsorption 

of corrosive sublimate 

by, 70 
BOHR, 4 
BOYS, 20 
BREDIG, 123 
Bromine, " solid," 178 
BROWN, ROBERT, 14, 107 
Brownian movement, 14, 107 
BUTSCHLI, 170, 180 
Butterfly wings, pattern on, 

190 



Camphor particles, move- 
ment of, 30 
Capillary analysis, 69 
chemistry, 12, 13 
rise, 21 

Capillary-active, 26, 27, 73 
Capillary-electrical pheno- 
mena, 75 

Capillary-inactive, 26, 28, 73 
Cardioid ultramicroscope, 

129 
Catalysis of gas reactions, 

50-2 

in solution, 71-5 
Cataphoresis, 75-8 
Cataphoretic migration velo- 
city, 76 

Cerebro-spinal fluid, 158 
Charcoal, adsorption of gases 

by, 43-6 

from solution, 59-62 
and amino-acid oxidation, 
73 74 



Charcoal, in box respirators 

of gas masks, 43 
and oxalic acid oxidation, 

72 
and phenylthiocarbamide 

oxidation, 72 

and phosgene synthesis, 52 
Clay, for casting, 167 
electrosmotic purification 

of, 88 

CLERK-MAXWELL, 23 
CLOWES, 167 
Coarse-disperse, 10 
Coagulation of sols, 116, 137, 

168 

Coagulum, 168 

Coal-dust explosions, 193, 194 
Colloidal ions, 153 

particles, molecular weight 

of, 151, 152 
shape of, 134, 171 
size of, 130, 150, 171 
state of aggregation of, 

133. 171 
solutions, 7 

osmotic pressure of, 151 

Colloidal-disperse, 10 

Colour adaptation of photo- 
chlorides, 200 

Coloured rock salt, 199 

Condensation method of pre- 
paring sols, 119 

Congo-red sol, 155 

Congo-rubin sol, 139 

Contact process for making 
sulphuric acid, 51 

COTTRELL process, 193 

Crystallisation, rate of, 94, 
98-102 



INDEX 



205 



Crystallites in metals, 42 



DEBYE, 105, 106, 133, 183, 
184 

Deturgescence, 175 

DEVAUX, 90 

Deviation in Brownian move- 
ment, 109 

Dialysis, 7, 114 

Dialysed iron, 122 

Dichroism, fluxional, 135 
of photochlorides, 200 

Diffusion in gels, 186 

Discharging of particles, 146 

Disintegration method of 
preparing sols, 123 

Disperse phase, 10 

Dispersion medium, 1 1 

Dispersion method of pre- 
paring sols, 123 

Displacement from surface, 
56,65 

DONNAN'S membrane equili- 
brium, 154 

Double-layer, electrical, 84, 

137. J 38 

Dyeing, 183-185 
Dynamite, 177 



EINSTEIN, 108, 109 
Elasticity of sols and gels, 

172, 173 
Electrokinetic potential, 78- 

84 

processes, 75 
Electrolyte, active, 144 
Electron, 5, 192 



Electrosmosis, 76 
Emulgent, 165 
Emulsions, 15, 165 
Emulsion colloids, 117 
Emulsoids, 117 
Exchange adsorption, 64, 65 
Explosions of dust, 193, 194 



Falling particles, potential 
due to, 77 

Fats and fatty acids, orienta- 
tion of molecules of, 

9i 
Fatty acids, surface tension 

of solutions of, 29 
Ferment actions, 75 
Ferments, adsorption of, 152 
Ferric hydroxide sols, 122, 

137. MO 

FISCHER, M. H., 181 
Flagella of bacteria, 145 
Flavones, adsorption of 

iodine by, 67 
Flocculation, 137 
Flotation of ores, 58, 196 
Flour, explosions of, 193, 194 
Fluorescence, 124, 125 
Fluxional birefringence, 134 

dichroism, 135 
Foams, 31, 194 
Foaming of beer wort, 195 
Fogs, London, 193 
Formic acid, decomposition 

catalysed by osmium, 

72 
Free energy, 33 

surface energy, 25, 39 
Froth, see Foams 



206 



COLLOIDAL CHEMISTRY 



Gall stones, 102 
Gas masks, 43 
Gels, 15, 1 68 
diffusion in, 186 
elasticity of, 172, 173 
inversion of, 176 
non-turgescible, 175-178 
turgescible, 175, 178, 179 
Gel pellicles at surfaces, 174 
Gelation, 169 
GIBBS' adsorption theorem, 

33, 49, 61 
Glittering of non-spherical 

particles, 134 
GOETHE'S theory of colour, 

126 

Gold crystals in gels, 187 
number, 157 
ruby-glass, 15, 198 
sols, 8, 9, 119-121, 124, 

139, 158 

preparation of, 119, 124 
unidisperse, 132, 133 
GOPPELSRODER, 69 
GRAHAM, 6, 9, 114 
Gypsum, increased solubility 
of minute particles of, 
53 

HABER, 49 

Haemoglobin, molecular 

weight of, 152 
Halation figures, 56 
Half -colloidal solutions, 114, 

152 

HARKINS, 91 
HATSCHEK, 187 
Heat of adsorption, 48 
of turgescence, 179 



HELMHOLTZ, 78, 79 
HOFF, VAN'T, 49, 109 
Homogeneous milk, 12 
Humus, adsorption by, 70 
Hydration of ions, 28 
Hydrocyanic acid inhibits 
catalytic oxidation, 74 
Hydrogen per oxide, decom- 
position catalysed by 
copper, 71 

decomposition by catalase 
and by colloidal pla- 
tinum, 75 
Hydrophilic, 57 
sols, 116 

interaction with hydro- 
phobic, 157 

peptization by, 164, 168 
stability of, 149 
Hydrophobic, 57 
sols, 116 

interaction with hydro- 
philic, 157 

with one another, 156 
stability of, 136 



Immersion ultramicroscope, 

128 
Inoculation of gold solutions, 

133 

Insecticides, 55 
Interfacial layers, 88-93 
tension, 17, 34 

between solids and 

liquids, 53 

Inversion of gels, 176 
Iodine, blue adsorption com- 
pounds of, 67, 68 



INDEX 



207 



Ions, 5, 192 

hydra tion of, 28 

in gases, 192 

Irregular series, 146, 147 
Irresoluble sols, 118 
Irreversible sols, 118 
Iso-electric point, 153 



Kaolin, adsorption by, 64, 65 
KATZ, 179 
KNIETSCH, 52 
KRUYT, 150 

KUNKEL, 198 



LANGMUIR, 49, 91, 163 
LAUE, VON, 104 
Leptinotarsa beetles, 180 
Leucolith process, 54 
LIESEGANG stratifications, 

186-190 

LOEB, JACQUES, 87 
Lyophilic sols, 117 
Lyophobic sols, 117 
Lyotropic series, 27 
Lysalbic acid, 158 



Mastic emulsions, 125 

MEISSNER, 41 

Membrane equilibrium, Don- 
nan's, 154 
potential, 87 

Metallic sols, 123 

Meteorites, 42 

Methylene blue, adsorption 
by kaolin, 64 

Microns, 131 



Milk, cataphoresis of, 75 

homogeneous, 12 

skin on, 174 
Mists, 191 

Molecular movement, Brown- 
ian, 14, 107 

weight of colloidal par- 
ticles, 151, 152 
Molecular-disperse, 10 
Molecules, 3 
Monodisperse sols, 132 
Multidisperse sols, 132 



Negative osmosis, 87 
NERNST'S potential, 79-83 
Nickel carbonyl, 52 
Nuclear formation, rate of, 
94-98 

Oedema, 181 

Oil films, thin, 37, 90 

Osmosis, 86 

negative, 87 
Osmotic pressure of sols, 151 

OSTWALD, WlLHELM, 1 88 

OSTWALD, WOLFGANG, 10 
Oxalic acid, catalytic oxi- 
dation of, 72 

Paper, adsorbent properties 

of, 68 
Pearls, 102 
Peat, drying of, 88 
Peptization, 147 

by hydrophilic sols, 164 
method of preparing sols, 
123 



208 



COLLOIDAL CHEMISTRY 



Permutites, 69 

PFEFFER'S cell, 87 

Phase-limit force, 79 

Phosgene, catalytic pre- 
paration of, 52 

Photochlorides, 200 

Photodichroism, 200 

Photographic plate, 201 

Photohalides, 199-201 

Platinum sols, 75, 123, 124, 
147 

POLANYI, 52 

Polar adsorption, 64, 66 
Polydisperse sols, 132 
Potential due to falling par- 
ticles, 77 

electrokinetic, 78-84 

Nernst's, 79-83 
Protalbic acid, 158 
Protective colloids, 157 
Protozoa, exoskeleton of, 58 



RAYLEIGH, LORD, 126 
Recrystallisation of metals, 

42 

Resoluble sols, 118 
Respiration, 74 
Reversal of charge, 146 
Reversible sols, 118 
RHUMBLER, 58 
Rock salt, coloured, 199 
ROENTGEN rays, see X-rays 
Ruby glass, 15, 198 
RUTHERFORD, 4 



SACHS-GEORGI reaction, 161 
Salts, coloured, 199 



Salvarsan, colloidal proper- 
ties of, 153 

Saponarin, adsorption of 
iodine by, 68 

Saturation value in adsorp- 
tion, 60 

SCHADE, l82 

SCHERRER, 105, 106, . 133, 
183, 184 

SCHWERIN, COUNT, 88 
Semi-colloids, 114, 152 
Sensitization, 159-163 

SlEDENTOPF, 7, 127, 198 

Silicic acid gels, 175-177 
Slit ultrarnicroscope, 127 
Smokes, 191-193 
SMOLUCHOWSKI, VON, 108, 109 
Soap films, 20, 196-197 
solutions, 152 

peptizmg action of, 164 
detergent action of, 167 
Soil, adsorption by, 70 
Sols, 7, 113-168 

elasticity of, 172, 173 
hydrophilic and hydro- 
phobic, 116 
interaction of, 155 
lyophilic and lyophobic, 117 
negative, 137, 139 
positive, 137, 140 
preparation of, 119-124 
reversible and irrever- 
sible, 118 

of alkali metals, 124 
of aluminium hydroxide, 

122, 137, 140 
of arsenious sulphide, 121, 

139 
of Congo-red, 155 



INDEX 



209 



Sols of Congo-rubin, 139 
of ferric hydroxide, 122, 

137. MO 
of gold, 8, 9. 119-121, 124, 

I39 158 
of mastic, 125 
of platinum, 123, 124, 147 
oS silver, 124, 158 
of vanadium pentoxide, 

123, 134, 135 
Sol-gel transformation, 169 
Solution molecules, 4 
Space-lattice, 5 
SPEK, 1 80 
Stalagmometer, 20 
STERN, R., 161 
Stibine, catalytic decom- 
position of, 50 
STRAUB, 70 

Streak method, Topler's, 54 
Streaks in vanadium pen- 
toxide sols, 135 
Stream dichroism, 135 
double refraction, 134 
potential, 77 
Submicrons, 131 
Sulphur trioxide, catalytic 

formation of, 51 
Superfused, see undercooled 
Surface energy, free, 25, 39 
tension, 17-30 

determination of, 20-21 
of capillary-active solu- 
tions, 28-30 

of capillary-inactive solu- 
tions, 27, 28 
of pure liquids, 18 
of solids, 37, 38 
of solutions, 26 



Surface-active, 26 
Surface-inactive, 26 
Suspensions, 15 
Suspension colloids, 117 
Suspensoids, 117 

SVEDBERG, 124 

Swelling, see Turgescence 



TAMMANN, 41, 95 
Tanning, 185 

electrosmotic, 88 
Tears of wine, 30 
TOPLER, 54 
TOWER, 1 80 
TRAUBE, I., 20, 29, 34, 61, 

62, 73, 93 
TRAUBE'S rule, 29, 34, 61, 

62, 73, 93 

Turgescence, 175-181 
Turgescible gels, 175, 178 
Turgidity pressure, 178 
TYNDALL effect, 125, 126 



Ultrafiltration, 131 
Ultramicroscopes, 8, 127-9 
Undercooled melts, 95 
Unidisperse sols, 132 
Urethanes, adsorbability and 
anticatalytic action, 73 
Urinary calculi, 102 



Valency, effect on electro- 
kinetic potential, 85 
effect on flocculation, 143 
Vanadium pentoxide sols, 
123, 134. 135 



2io COLLOIDAL CHEMISTRY 

Vapour pressure of small X-rays used to distinguish 

drops, 40 crystalline and amor- 

Veratrine, adsorption by phous states, 104-7, 

Aplysia heart, 70 133 

VOLMER, 99 ' ionisation of gases by, 192 



WARBURG, O., 74 Yeast cells, adsorptioft of 

Washing by soaps, 167 phenol by, 70 

WASSERMANN reaction, 161 

WEIGERT, 200 

Wetting, 1 8, 21, 54 Zeoliths, 69, 70 

WIEMER, CHR., 108 ZSIGMONDY, 7, 119, 127, 128, 

WlLLSTATTER, 152 132, 144, 157, 17!, 198 



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